Formula to forecast. (Moving Average - I think)

[lmhudson]

I have the following data on my spreadsheet. The Grey rows are actual data I have collected so far. The white cells below is data I have made up to reflect what I "KNOW" the trend will look like based on my experiance. (Over time I will capture more actual data and replace this)

Plotting these on a graph (XY), shows the pattern seemingly fits the "Moving Average" trend.

What I want to do is find a formaula for column "F"
I.e. find new "Y" for New value in "X".

Ive tried the forecast and trend function, both of which seem too linear.

Can anyone help with this formula ?

Excel Workbook
	B	C	D	E	F
2	Known X's	Known Y's		New X	Forecasted Y
3	10170	38.25		1000
4	9566	37.83		1500
5	9401	37.77		2000
6	8855	35.75		2500
7	8684	37.63		3000
8	8625	37.15		3500
9	7458	36.23		4000
10	7108	35.16		4500
11	6988	34.23		5000
12	6971	33.24		5500
13	6900	34.74		6000
14	6748	34.75		6500
15	6200	33.01244		7000
16	5900	30.53651		7500
17	5600	27.48286		8000
18	5300	24.0475		8500
19	5000	20.44037		9000
20	4700	16.86331		9500
21	4400	13.65928		10000
22	4100	10.85913		10500
23	3800	8.470119		11000
24	3500	6.479641		11500
25	3200	4.859731		12000
26	2900	3.571902
27	2600	2.57177
28	2300	1.813098
29	2000	1.251037
30	1700	0.84445
Sheet2

Excel 2003

Cell Formulas
Range	Formula
E4	=E3+500
E5	=E4+500
E6	=E5+500
E7	=E6+500
E8	=E7+500
E9	=E8+500
E10	=E9+500
E11	=E10+500
E12	=E11+500
E13	=E12+500
E14	=E13+500
E15	=E14+500
E16	=E15+500
E17	=E16+500
E18	=E17+500
E19	=E18+500
E20	=E19+500
E21	=E20+500
E22	=E21+500
E23	=E22+500
E24	=E23+500
E25	=E24+500

 

[lmhudson]

Update:
I've been looking at moving average formulas and am thinking this may not be what I need.
If what I'm looking at is correct this is only looking at Known Y's, which may be why the trend fits so well on the graph?

Anyway, still looking for the formula to forecast Y's from my data trend, whatever the right way is !