To do this, you must first set up a sub-budget for your sub-contract. (Go to the Sub Budgets tab, click Add Sub Budget, enter a working title for the sub-contract budget, and then select Subcontract Budget from the Budget Type drop down menu.)
Navigate to the Salaries and Wages line of your subcontract budget worksheet, add a person to your sub-budget, and then click to the first period. At the bottom of the Personnel Details window, in the Benefit % textbox, enter the benefit rate you want to apply to the sub-contractor. You need to enter the custom benefit rate in this textbox for each budget period.