Calculate using only the last X cells of data in an array (monthly mutual fund performance figures)

[j3ff]

I'm trying to create a formula to calculate the performance for a mutual fund over the last year (a product of the last 12 monthly returns in an array). Here's a simplified example.

B1:N1 contain Jan, Feb, Mar...Dec
A2:A3 show year text 2010, 2011

The array of data is B2:M3. The data is monthly percentage returns but not in percentage format i.e. 2% is entered as 2 (rather than .02).

N2 gives the total return for the year 2010 with this array formula:
{=PRODUCT(B2:M2/100+1)-1}

N3 is the same formula adjusted down one row.

Row 2 contains all 12 data points but row 3 only has 5 data points as there is only data up to May 2011.

I need a formula to look at this array and determine the last 12 periods and then apply the formula to them to determine the cumulative performance. When I get the June 2011 performance figure and enter data into G3, I need it to adjust that 12 month period automatically.

In this example it would need to find the product of G2:M2 and B3:F3. When I enter June 2011 it would then need to find the product of H2:M2 and B3:G3 etc. etc.

Thanks in advance for any help! I hope my explanation is clear.

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[j3ff]

bump

 

[j3ff]

bump - if this isn't possible please just let me know. I was thinking a workaround was to transpose the data to another sheet and skip the array technicality...I have seen a solution for using "Last X data points" - just not in an array.