One way:

Assuming your "Mar05" entry is in C1, and that you want the values in
succeeding months to be equal:

C2: =ROUND(($M2-SUM($A2:B2))/(COLUMN($M2)-COLUMN()),2)

and copy across to L2. Then when you enter Mar05 data, the cells to the
right will adjust.

If you need a growth formula, rather than linear, what assumptions are
you making about the rate of growth?

In article ,
"Hella" wrote:

I need help in forecasting. so any help will be highly
appreciated.

I have a data as follows:
Jan05 Feb05 Mar05 Apr05 May05 June05...... Dec05 Total
103 107 ... ... ... .... 1700

Based on Jan05 and Feb 05 data I need to fill up Mar 05
to Dec05 data to get a pre determined total of 1700. Is
there any quick way or formula or function to solve this??
Thanks for your help.