If you pay an employee using an Ace Payroll extra emolument, you cannot overwrite the tax deduction with your own value.

The whole point of the extra emolument routine is to allow you to make a non standard payment, but have the program calculate the tax using the very complicated formula specified by the IRD.

If you want to be able to change the tax, do not use the extra emolument routine.

Enter the gross amount some other way, for example by using an allowance or entering the amount as a salary. Either of these methods will acheive the same result, but you will be able to change the tax deduction.