Ideas for averages

[ID015]

I need some ideas on how to calculate incremental estimates...
For example, monthly consumption of units or whatever:

Month 1=150
Month 2=200 (200-150=50)
Month 3=215 (215-200=15)
Month 4 estimate = 247 (215+((50+15)/2))

I would like to keep two tables, one with actual and other with estimates and with the estimates updating as the actuals are entered.

So if Month 4 happens to be 270, estimate for Month 5 is 310.

I can do that with having a hidden table calculating the differences between previous months, etc but maybe there's a better way!

[shg]

Maybe ...

Row\Col	A	B	C	D
1	Month	Actual	Fcst
2	1	150
3	2	200
4	3	215	250.0	C4: =FORECAST(A4, B$2:B3, A$2:A3)
5	4		253.3
6	5		285.8

[ID015]

TBH - I haven't used FORECAST function before but I can learn I did have a look at it before but not sure it was clicking... still doesn't fully.
Whatever in the formula for Column A - how's that used? I don't have a column A, I have one with months (Jan, Feb, Mar, etc) and for A4 I could use ROW, but for the last part A$2:A3 I don't know how it'd work.

TY!

[shg]

Without col A:

Row\Col	A	B	C
1	Actual	Fcst
2	150
3	200
4	215	250.0	B4: =FORECAST(ROW(), A$2:A3, ROW(A$2:A3))
5		253.3
6		285.8