Adjusting Distribution

Hello

First of all I am using Exel 2003 and have a mediocure knowledge of the program

I have two rows, one is labelled actual and one is labeled forcast. What I need is that say my actual number differece from my forcast . I want it to take the data and update the next cell in the actual. What I mean is


say I have

Forecast will be row A, and Actual will be Row B

And say A1 is 5 and B 1 is 1, and A2 is 3

is there a formula I can use to calculate the difference between A1 and B1 and then project it to B2 and adding still allowing me to enter a value for B2 witch isnt hard coded and will erase my formula

and It will be a distribution of say 100 and in the end both A and B should add up to 100 but maybe with a different distribution.




Thanks in Advance