I think the new "Percentage of Disposable Wages" calculation method would do this.  When you choose this, a Deduction button is available to select which deductions should be subtracted from gross wages.

Thanks, Beth

I believe this deduction calculation method is actually not based on Gross.  It does not appear to calculate the deduction until after Payroll Tax Calculation and it appears to be based on Net, less the deductions chosen to exclude. 

Please confirm that I am understanding this correctly and if there is a way to work around this in order to have the calculation based on Gross, less deduction chosen to exclude.  Thanks so much!

Rats.  So close, and yet so far....