So, my base amount is gross pay.
My first line item is the remainder to payroll expense
My next line item is the percentage (negative percent) for FICA withholding, which is added to base (because it is a negative #, the amount of base is reduced)
Next I have fixed amounts for federal and state withholdings (because I'm lazy and my the base salary doesn't change); each of those is a negative number which is added to the base amount (again, it reduces the base pay)
Finally, since remainder only records a debit, I have a -100% of base for the check amount that is credited to my bank account (net pay)

So, there is an entry for gross pay, a deduction for FICA (percentage), deductions for state and federal income taxes, then the net amount is calculated as a negative 100% of base (which has been reduced by deductions).

I hope this is clear. It took me a while to figure this out.

Screen shot of quick entry