How to sort groups by derived formula

My report contains two groups. First grouping is by department and second group is by pay_code (for example, OT, OT1, OT5 are overtime paycodes and RG, RG1, RG5 are regular hour paycodes). Each group at the department level contains a running total of overtime and regulartime wages (wage_amount is a database field for each record). I've inserted a formula in the department group footer: runningTotalOvertimeWages/runningTotalRegularTimeWages.

There are about 70 department groups however I only want to show the 10 departments that have the highest overtime rate (result of formula described above.) I've read the docs for the Group Sort Expert however this option is not enabled because I'm using running totals and not summaries. Using a summary instead of a running total doesn't produce expected results (because second group is defined conditionally based on value of paycode.)

Is there another approach I can use?