This is probably simple, but I have a report with line-item sales data that is totaled/grouped by item, by customer, by sales rep. The columns are 2008 sales and 2009 sales. I need to calculate the % change from 08 to 09 for each summary group. Like this:

.........................Sales08....Sales09....%Diff
Item
..Customer
....Rep
......Report

I have the formula to do the calculation but I'm not sure how to create the field for each group footer to do the calculation. I've tried it as a formula field and a SQL field and I can't get it. I'm using CR v10. Thanks.