Dashboard Report - calculating % on Group Sum values

Hi all,

I'm trying to figure out how to insert a field within my report's Group Footer that would perform a calculation on other Group Sum fields. In my Group Footer, I'm summing up Contract Amount and Contract Margin fields from the Detail band. I would like to calculate the Margin % based on these sums and have no idea how this is done. 

Any advice would be greatly appreciated!

Cosimo