I want my employee to have a net paycheck of a certain amount, after deductions

Desktop Payroll has a Gross-up feature which reverse-calculates gross pay and taxes based on a net amount you enter, like $100 for a bonus.

I don't think Online Payroll can do that.

Using online payroll you will have to play around with the numbers to gross it up. If you were using desktop payroll it offers a convenient feature to enter net and it calculates gross. From there you would simply divide by hours.

In a nutshell (Algebra I, 8th grade)

GROSS PAY - FWT - %DEDUCTIONS×GROSS =NET.

As a tentative, look up what FWT should be close to from the IRS tables. Add that to the net. Then divide this by the reduced ,% from the left side. Deductions are FICA plus State plus Local. If 10% you divide the right side by .9 

Example you want net of 1000. You estimate FWT at 100, taxes total 10% (FICA alone  is 7.65%)

GR - 100 - .1GR = 1000

Combining

.9GR = 1100

GR = 1222.22