Please don't make agreements based on net wage. It's too hard to make sure someone actually gets that amount, since it's dependent on so many factors, like contract type, if the contract is indefinite or not, position, being a skilled worker or not, social premiums, pension, holiday allowance, other benefits, working during public holidays, other premiums, etc etc.
If you still want to do this, you can estimate a gross wage, tell them to sign the contract, go to Pay, and see what net wage is showing on the payslip of the employee. That is the net salary they will get. If it's too high or low, go back to the salary of the employee, adjust it with an effective from date on the first day of their contract, apply it, and see what the new net wage is in Pay.