{Excel 2003}

Each Row is revenue by customer
Each column is a month {jan-07, feb-07, mar-07}
We have historical revenue for each month.
In a seperate column we have an identifier when the contract changes {JUL}.
(Different for each customer)
In another column we have a percent inc/dec/ expected over existing {10%}

How do I set it up so in the cell where I want an ouput. I have a formula
that says if this month or greater fill in a value that is x% more/less than
the average of the past revenue amounts?