There are a couple of options that come to mind.

1. If you google 'calculate net pay' there are several sites that have an online calculator.  You may want to try a couple to make sure the net pay is the same. 

2. Another option in Sage HRMS, is to create a timecard for the employee, calculate payroll for this one person, look at Payroll pre-check register report to identify the net amount, then delete the employees timecard.  If there are no other payrolls waiting to be processed, you can go to Calculate payroll and delete.