This is not an employee deduction so I keep it away from payroll. Each month I take the full amount recorded in the Wages accounts, apply the percentage and create a journal entry to set up the payable. The Other Expenses tab in the employee record could be utilized but only if the same amount is paid each period and the percentage could then be shown as a dollar value. I find it easier to make one calculation per month compared to a calculation per employee and remembering when ever the pay changes.