If I've understood correctly
a) Incentive Payments = Base Salary * x%, configured in Labor Allocation Rules.
b) And you would like to achieve: Payroll Taxes = Incentive Payments * y%.
If b) is correct, you could add this rule in Labor Allocation Rules: Payroll Taxes = Base Salary * x% * y%
#ApptioPlanning(ITP/ITFMF)