Hi JenDPT,
I see you're trying to set up payroll giving in MYOB Business Payroll. While I can't give you specific financial advice, I can certainly guide you a bit. You're on the right track with deducting the donation amount from the employee's salary and working out the tax credit. Here are a couple of links that might be useful for you:
I would also recommend that you speak with your accountant for further assistance.
Feel free to get in touch if there's anything else I can assist you with.
Cheers,
Princess