Sum Arbitrary Level Rollups

I have a report with a custom calculation that is summing 2 levels together. For my Gross Margin % calculation, the calculation is adding together the Gross Margin percentages rather than calculating on the aggregate. Is there a way to have the report calculate using the aggregate?

This can be done by using the levelrollup function instead of adding together the 2 levels in the Custom Calculation. This function can be used to perform arbitrary level rollup calculations in reports.