The Payroll module can be used to calculate a supplemental tax rate on the checks in a Computer Check batch. Create a FED and/or STATE tax table record with a status of ST using the Tax Tables Maintenance window (PR> Maintenance> Tax Tables> Create a new tax table entry). The tax table record should have one rate line with a flat percentage amount and no federal exclusion, minimum income, standard deduction, tax credit or other deduction selected.
When you want to process a Computer Check batch (PR> Computer Checks), check the Use Supplemental Tax Rate toggle on the employee timecards (PR> Computer Checks> Timecards> Select an employee on the Selection tab> Open the Check tab> Check the Use Supplemental Tax toggle). Taxes will be withheld using the flat percentage amount set up on the tax table on the employee record.