Hi Lori and welcome to the board.
One possible solution would be to use Linear Interpolation. There is an article here which describes its use:
http://support.microsoft.com/kb/214096
The basic formula is:
=(end-start)/(ROW(end)-ROW(start))
In the sample below the end row is B16 and the start row is B4.
Sheet1
<TABLE style="BACKGROUND-COLOR: #ffffff; PADDING-LEFT: 2pt; PADDING-RIGHT: 2pt; FONT-FAMILY: Arial,Arial; FONT-SIZE: 10pt" border=1 cellSpacing=0 cellPadding=0><COLGROUP><COL style="WIDTH: 30px; FONT-WEIGHT: bold"><COL style="WIDTH: 43px"><COL style="WIDTH: 142px"><COL style="WIDTH: 64px"></COLGROUP><TBODY><TR style="TEXT-ALIGN: center; BACKGROUND-COLOR: #cacaca; FONT-SIZE: 8pt; FONT-WEIGHT: bold"><TD></TD><TD>A</TD><TD>B</TD><TD>C</TD></TR><TR style="HEIGHT: 17px"><TD style="TEXT-ALIGN: center; BACKGROUND-COLOR: #cacaca; FONT-SIZE: 8pt">1</TD><TD></TD><TD style="FONT-WEIGHT: bold">Potentail Market size</TD><TD style="TEXT-ALIGN: right; FONT-WEIGHT: bold">5000000</TD></TR><TR style="HEIGHT: 17px"><TD style="TEXT-ALIGN: center; BACKGROUND-COLOR: #cacaca; FONT-SIZE: 8pt">2</TD><TD></TD><TD style="FONT-WEIGHT: bold">Subscription</TD><TD style="TEXT-ALIGN: right; FONT-WEIGHT: bold">11.99</TD></TR><TR style="HEIGHT: 17px"><TD style="TEXT-ALIGN: center; BACKGROUND-COLOR: #cacaca; FONT-SIZE: 8pt">3</TD><TD style="FONT-WEIGHT: bold"></TD><TD style="FONT-WEIGHT: bold"></TD><TD style="FONT-WEIGHT: bold"></TD></TR><TR style="HEIGHT: 17px"><TD style="TEXT-ALIGN: center; BACKGROUND-COLOR: #cacaca; FONT-SIZE: 8pt">4</TD><TD>Month</TD><TD style="TEXT-ALIGN: right; BACKGROUND-COLOR: #99ccff">0</TD><TD>Income</TD></TR><TR style="HEIGHT: 17px"><TD style="TEXT-ALIGN: center; BACKGROUND-COLOR: #cacaca; FONT-SIZE: 8pt">5</TD><TD style="TEXT-ALIGN: right">1</TD><TD style="TEXT-ALIGN: right; BACKGROUND-COLOR: #ffff00; COLOR: #333333">2083</TD><TD style="TEXT-ALIGN: right">0</TD></TR><TR style="HEIGHT: 17px"><TD style="TEXT-ALIGN: center; BACKGROUND-COLOR: #cacaca; FONT-SIZE: 8pt">6</TD><TD style="TEXT-ALIGN: right">2</TD><TD style="TEXT-ALIGN: right; BACKGROUND-COLOR: #ffff00; COLOR: #333333">4167</TD><TD style="TEXT-ALIGN: right">49,958</TD></TR><TR style="HEIGHT: 17px"><TD style="TEXT-ALIGN: center; BACKGROUND-COLOR: #cacaca; FONT-SIZE: 8pt">7</TD><TD style="TEXT-ALIGN: right">3</TD><TD style="TEXT-ALIGN: right; BACKGROUND-COLOR: #ffff00; COLOR: #333333">6250</TD><TD style="TEXT-ALIGN: right">74,938</TD></TR><TR style="HEIGHT: 17px"><TD style="TEXT-ALIGN: center; BACKGROUND-COLOR: #cacaca; FONT-SIZE: 8pt">8</TD><TD style="TEXT-ALIGN: right">4</TD><TD style="TEXT-ALIGN: right; BACKGROUND-COLOR: #ffff00; COLOR: #333333">8333</TD><TD style="TEXT-ALIGN: right">99,917</TD></TR><TR style="HEIGHT: 17px"><TD style="TEXT-ALIGN: center; BACKGROUND-COLOR: #cacaca; FONT-SIZE: 8pt">9</TD><TD style="TEXT-ALIGN: right">5</TD><TD style="TEXT-ALIGN: right; BACKGROUND-COLOR: #ffff00; COLOR: #333333">10417</TD><TD style="TEXT-ALIGN: right">124,896</TD></TR><TR style="HEIGHT: 17px"><TD style="TEXT-ALIGN: center; BACKGROUND-COLOR: #cacaca; FONT-SIZE: 8pt">10</TD><TD style="TEXT-ALIGN: right">6</TD><TD style="TEXT-ALIGN: right; BACKGROUND-COLOR: #ffff00; COLOR: #333333">12500</TD><TD style="TEXT-ALIGN: right">149,875</TD></TR><TR style="HEIGHT: 17px"><TD style="TEXT-ALIGN: center; BACKGROUND-COLOR: #cacaca; FONT-SIZE: 8pt">11</TD><TD style="TEXT-ALIGN: right">7</TD><TD style="TEXT-ALIGN: right; BACKGROUND-COLOR: #ffff00; COLOR: #333333">14583</TD><TD style="TEXT-ALIGN: right">174,854</TD></TR><TR style="HEIGHT: 17px"><TD style="TEXT-ALIGN: center; BACKGROUND-COLOR: #cacaca; FONT-SIZE: 8pt">12</TD><TD style="TEXT-ALIGN: right">8</TD><TD style="TEXT-ALIGN: right; BACKGROUND-COLOR: #ffff00; COLOR: #333333">16667</TD><TD style="TEXT-ALIGN: right">199,833</TD></TR><TR style="HEIGHT: 17px"><TD style="TEXT-ALIGN: center; BACKGROUND-COLOR: #cacaca; FONT-SIZE: 8pt">13</TD><TD style="TEXT-ALIGN: right">9</TD><TD style="TEXT-ALIGN: right; BACKGROUND-COLOR: #ffff00; COLOR: #333333">18750</TD><TD style="TEXT-ALIGN: right">224,813</TD></TR><TR style="HEIGHT: 17px"><TD style="TEXT-ALIGN: center; BACKGROUND-COLOR: #cacaca; FONT-SIZE: 8pt">14</TD><TD style="TEXT-ALIGN: right">10</TD><TD style="TEXT-ALIGN: right; BACKGROUND-COLOR: #ffff00; COLOR: #333333">20833</TD><TD style="TEXT-ALIGN: right">249,792</TD></TR><TR style="HEIGHT: 17px"><TD style="TEXT-ALIGN: center; BACKGROUND-COLOR: #cacaca; FONT-SIZE: 8pt">15</TD><TD style="TEXT-ALIGN: right">11</TD><TD style="TEXT-ALIGN: right; BACKGROUND-COLOR: #ffff00; COLOR: #333333">22917</TD><TD style="TEXT-ALIGN: right">274,771</TD></TR><TR style="HEIGHT: 17px"><TD style="TEXT-ALIGN: center; BACKGROUND-COLOR: #cacaca; FONT-SIZE: 8pt">16</TD><TD style="TEXT-ALIGN: right">12</TD><TD style="TEXT-ALIGN: right; BACKGROUND-COLOR: #99ccff">25000</TD><TD style="TEXT-ALIGN: right">299,750</TD></TR></TBODY></TABLE>
<TABLE style="BORDER-BOTTOM-STYLE: groove; BORDER-BOTTOM-COLOR: #00ff00; BORDER-RIGHT-STYLE: groove; BACKGROUND-COLOR: #fffcf9; BORDER-TOP-COLOR: #00ff00; FONT-FAMILY: Arial; BORDER-TOP-STYLE: groove; COLOR: #000000; BORDER-RIGHT-COLOR: #00ff00; FONT-SIZE: 10pt; BORDER-LEFT-STYLE: groove; BORDER-LEFT-COLOR: #00ff00"><TBODY><TR><TD>
Spreadsheet Formulas</TD></TR><TR><TD><TABLE style="FONT-FAMILY: Arial; FONT-SIZE: 9pt" border=1 cellSpacing=0 cellPadding=2><TBODY><TR style="BACKGROUND-COLOR: #cacaca; FONT-SIZE: 10pt"><TD>Cell</TD><TD>Formula</TD></TR><TR><TD>B5</TD><TD>=+B4+($B$16-$B$4)/(ROW
($B$16)-ROW
($B$4))</TD></TR><TR><TD>B6</TD><TD>=+B5+($B$16-$B$4)/(ROW
($B$16)-ROW
($B$4))</TD></TR><TR><TD>C6</TD><TD>=+B6*$C$2</TD></TR><TR><TD>B7</TD><TD>=+B6+($B$16-$B$4)/(ROW
($B$16)-ROW
($B$4))</TD></TR><TR><TD>C7</TD><TD>=+B7*$C$2</TD></TR><TR><TD>B8</TD><TD>=+B7+($B$16-$B$4)/(ROW
($B$16)-ROW
($B$4))</TD></TR><TR><TD>C8</TD><TD>=+B8*$C$2</TD></TR><TR><TD>B9</TD><TD>=+B8+($B$16-$B$4)/(ROW
($B$16)-ROW
($B$4))</TD></TR><TR><TD>C9</TD><TD>=+B9*$C$2</TD></TR><TR><TD>B10</TD><TD>=+B9+($B$16-$B$4)/(ROW
($B$16)-ROW
($B$4))</TD></TR><TR><TD>C10</TD><TD>=+B10*$C$2</TD></TR><TR><TD>B11</TD><TD>=+B10+($B$16-$B$4)/(ROW
($B$16)-ROW
($B$4))</TD></TR><TR><TD>C11</TD><TD>=+B11*$C$2</TD></TR><TR><TD>B12</TD><TD>=+B11+($B$16-$B$4)/(ROW
($B$16)-ROW
($B$4))</TD></TR><TR><TD>C12</TD><TD>=+B12*$C$2</TD></TR><TR><TD>B13</TD><TD>=+B12+($B$16-$B$4)/(ROW
($B$16)-ROW
($B$4))</TD></TR><TR><TD>C13</TD><TD>=+B13*$C$2</TD></TR><TR><TD>B14</TD><TD>=+B13+($B$16-$B$4)/(ROW
($B$16)-ROW
($B$4))</TD></TR><TR><TD>C14</TD><TD>=+B14*$C$2</TD></TR><TR><TD>B15</TD><TD>=+B14+($B$16-$B$4)/(ROW
($B$16)-ROW
($B$4))</TD></TR><TR><TD>C15</TD><TD>=+B15*$C$2</TD></TR><TR><TD>B16</TD><TD>=+C1*0.005</TD></TR><TR><TD>C16</TD><TD>=+B16*$C$2</TD></TR></TBODY></TABLE></TD></TR></TBODY></TABLE>
NB For the second month the end row and start row changes to B28 and B16 respectively. Note how these cells contain your forecast values for the years end.
Sheet1
<TABLE style="BACKGROUND-COLOR: #ffffff; PADDING-LEFT: 2pt; PADDING-RIGHT: 2pt; FONT-FAMILY: Arial,Arial; FONT-SIZE: 10pt" border=1 cellSpacing=0 cellPadding=0><COLGROUP><COL style="WIDTH: 30px; FONT-WEIGHT: bold"><COL style="WIDTH: 43px"><COL style="WIDTH: 142px"><COL style="WIDTH: 64px"></COLGROUP><TBODY><TR style="TEXT-ALIGN: center; BACKGROUND-COLOR: #cacaca; FONT-SIZE: 8pt; FONT-WEIGHT: bold"><TD></TD><TD>A</TD><TD>B</TD><TD>C</TD></TR><TR style="HEIGHT: 17px"><TD style="TEXT-ALIGN: center; BACKGROUND-COLOR: #cacaca; FONT-SIZE: 8pt">16</TD><TD style="TEXT-ALIGN: right">12</TD><TD style="TEXT-ALIGN: right; BACKGROUND-COLOR: #99ccff">25000</TD><TD style="TEXT-ALIGN: right">299,750</TD></TR><TR style="HEIGHT: 17px"><TD style="TEXT-ALIGN: center; BACKGROUND-COLOR: #cacaca; FONT-SIZE: 8pt">17</TD><TD style="TEXT-ALIGN: right">13</TD><TD style="TEXT-ALIGN: right; BACKGROUND-COLOR: #ffff00; COLOR: #333333">27083</TD><TD style="TEXT-ALIGN: right">324,729</TD></TR><TR style="HEIGHT: 17px"><TD style="TEXT-ALIGN: center; BACKGROUND-COLOR: #cacaca; FONT-SIZE: 8pt">18</TD><TD style="TEXT-ALIGN: right">14</TD><TD style="TEXT-ALIGN: right; BACKGROUND-COLOR: #ffff00; COLOR: #333333">29167</TD><TD style="TEXT-ALIGN: right">349,708</TD></TR><TR style="HEIGHT: 17px"><TD style="TEXT-ALIGN: center; BACKGROUND-COLOR: #cacaca; FONT-SIZE: 8pt">19</TD><TD style="TEXT-ALIGN: right">15</TD><TD style="TEXT-ALIGN: right; BACKGROUND-COLOR: #ffff00; COLOR: #333333">31250</TD><TD style="TEXT-ALIGN: right">374,688</TD></TR><TR style="HEIGHT: 17px"><TD style="TEXT-ALIGN: center; BACKGROUND-COLOR: #cacaca; FONT-SIZE: 8pt">20</TD><TD style="TEXT-ALIGN: right">16</TD><TD style="TEXT-ALIGN: right; BACKGROUND-COLOR: #ffff00; COLOR: #333333">33333</TD><TD style="TEXT-ALIGN: right">399,667</TD></TR><TR style="HEIGHT: 17px"><TD style="TEXT-ALIGN: center; BACKGROUND-COLOR: #cacaca; FONT-SIZE: 8pt">21</TD><TD style="TEXT-ALIGN: right">17</TD><TD style="TEXT-ALIGN: right; BACKGROUND-COLOR: #ffff00; COLOR: #333333">35417</TD><TD style="TEXT-ALIGN: right">424,646</TD></TR><TR style="HEIGHT: 17px"><TD style="TEXT-ALIGN: center; BACKGROUND-COLOR: #cacaca; FONT-SIZE: 8pt">22</TD><TD style="TEXT-ALIGN: right">18</TD><TD style="TEXT-ALIGN: right; BACKGROUND-COLOR: #ffff00; COLOR: #333333">37500</TD><TD style="TEXT-ALIGN: right">449,625</TD></TR><TR style="HEIGHT: 17px"><TD style="TEXT-ALIGN: center; BACKGROUND-COLOR: #cacaca; FONT-SIZE: 8pt">23</TD><TD style="TEXT-ALIGN: right">19</TD><TD style="TEXT-ALIGN: right; BACKGROUND-COLOR: #ffff00; COLOR: #333333">39583</TD><TD style="TEXT-ALIGN: right">474,604</TD></TR><TR style="HEIGHT: 17px"><TD style="TEXT-ALIGN: center; BACKGROUND-COLOR: #cacaca; FONT-SIZE: 8pt">24</TD><TD style="TEXT-ALIGN: right">20</TD><TD style="TEXT-ALIGN: right; BACKGROUND-COLOR: #ffff00; COLOR: #333333">41667</TD><TD style="TEXT-ALIGN: right">499,583</TD></TR><TR style="HEIGHT: 17px"><TD style="TEXT-ALIGN: center; BACKGROUND-COLOR: #cacaca; FONT-SIZE: 8pt">25</TD><TD style="TEXT-ALIGN: right">21</TD><TD style="TEXT-ALIGN: right; BACKGROUND-COLOR: #ffff00; COLOR: #333333">43750</TD><TD style="TEXT-ALIGN: right">524,563</TD></TR><TR style="HEIGHT: 17px"><TD style="TEXT-ALIGN: center; BACKGROUND-COLOR: #cacaca; FONT-SIZE: 8pt">26</TD><TD style="TEXT-ALIGN: right">22</TD><TD style="TEXT-ALIGN: right; BACKGROUND-COLOR: #ffff00; COLOR: #333333">45833</TD><TD style="TEXT-ALIGN: right">549,542</TD></TR><TR style="HEIGHT: 17px"><TD style="TEXT-ALIGN: center; BACKGROUND-COLOR: #cacaca; FONT-SIZE: 8pt">27</TD><TD style="TEXT-ALIGN: right">23</TD><TD style="TEXT-ALIGN: right; BACKGROUND-COLOR: #ffff00; COLOR: #333333">47917</TD><TD style="TEXT-ALIGN: right">574,521</TD></TR><TR style="HEIGHT: 17px"><TD style="TEXT-ALIGN: center; BACKGROUND-COLOR: #cacaca; FONT-SIZE: 8pt">28</TD><TD style="TEXT-ALIGN: right">24</TD><TD style="TEXT-ALIGN: right; BACKGROUND-COLOR: #99ccff">50000</TD><TD style="TEXT-ALIGN: right">599,500</TD></TR></TBODY></TABLE>
<TABLE style="BORDER-BOTTOM-STYLE: groove; BORDER-BOTTOM-COLOR: #00ff00; BORDER-RIGHT-STYLE: groove; BACKGROUND-COLOR: #fffcf9; BORDER-TOP-COLOR: #00ff00; FONT-FAMILY: Arial; BORDER-TOP-STYLE: groove; COLOR: #000000; BORDER-RIGHT-COLOR: #00ff00; FONT-SIZE: 10pt; BORDER-LEFT-STYLE: groove; BORDER-LEFT-COLOR: #00ff00"><TBODY><TR><TD>
Spreadsheet Formulas</TD></TR><TR><TD><TABLE style="FONT-FAMILY: Arial; FONT-SIZE: 9pt" border=1 cellSpacing=0 cellPadding=2><TBODY><TR style="BACKGROUND-COLOR: #cacaca; FONT-SIZE: 10pt"><TD>Cell</TD><TD>Formula</TD></TR><TR><TD>B16</TD><TD>=+C1*0.005</TD></TR><TR><TD>C16</TD><TD>=+B16*$C$2</TD></TR><TR><TD>B17</TD><TD>=+B16+($B$28-$B$16)/(ROW
($B$28)-ROW
($B$16))</TD></TR><TR><TD>C17</TD><TD>=+B17*$C$2</TD></TR><TR><TD>B18</TD><TD>=+B17+($B$28-$B$16)/(ROW
($B$28)-ROW
($B$16))</TD></TR><TR><TD>C18</TD><TD>=+B18*$C$2</TD></TR><TR><TD>B19</TD><TD>=+B18+($B$28-$B$16)/(ROW
($B$28)-ROW
($B$16))</TD></TR><TR><TD>C19</TD><TD>=+B19*$C$2</TD></TR><TR><TD>B20</TD><TD>=+B19+($B$28-$B$16)/(ROW
($B$28)-ROW
($B$16))</TD></TR><TR><TD>C20</TD><TD>=+B20*$C$2</TD></TR><TR><TD>B21</TD><TD>=+B20+($B$28-$B$16)/(ROW
($B$28)-ROW
($B$16))</TD></TR><TR><TD>C21</TD><TD>=+B21*$C$2</TD></TR><TR><TD>B22</TD><TD>=+B21+($B$28-$B$16)/(ROW
($B$28)-ROW
($B$16))</TD></TR><TR><TD>C22</TD><TD>=+B22*$C$2</TD></TR><TR><TD>B23</TD><TD>=+B22+($B$28-$B$16)/(ROW
($B$28)-ROW
($B$16))</TD></TR><TR><TD>C23</TD><TD>=+B23*$C$2</TD></TR><TR><TD>B24</TD><TD>=+B23+($B$28-$B$16)/(ROW
($B$28)-ROW
($B$16))</TD></TR><TR><TD>C24</TD><TD>=+B24*$C$2</TD></TR><TR><TD>B25</TD><TD>=+B24+($B$28-$B$16)/(ROW
($B$28)-ROW
($B$16))</TD></TR><TR><TD>C25</TD><TD>=+B25*$C$2</TD></TR><TR><TD>B26</TD><TD>=+B25+($B$28-$B$16)/(ROW
($B$28)-ROW
($B$16))</TD></TR><TR><TD>C26</TD><TD>=+B26*$C$2</TD></TR><TR><TD>B27</TD><TD>=+B26+($B$28-$B$16)/(ROW
($B$28)-ROW
($B$16))</TD></TR><TR><TD>C27</TD><TD>=+B27*$C$2</TD></TR><TR><TD>B28</TD><TD>=+C1*0.01</TD></TR><TR><TD>C28</TD><TD>=+B28*$C$2</TD></TR></TBODY></TABLE></TD></TR></TBODY></TABLE>
Repeating this pattern for the remaining years:
B29=B28+($B$40-$B$28)/(ROW($B$40)-ROW($B$28))
B41=B40+($B$52-$B$40)/(ROW($B$52)-ROW($B$40))
B53=B52+($B$64-$B$52)/(ROW($B$64)-ROW($B$52))
And drag the formula down for the intervening months.
Hope this helps,
Bertie