Not a formula. You need a manual. You either got it on paper or CD when you bought the motherboard, or can download it from the web if you know manufacturer and type. Use google to find it, if necessary. Use search terms like download asus p5gd2 manual and you'll find it. Or go to the manufacturers website directly and search for support or download links.
Hope this helps.
I have a short question. How can I tell how much memory is the max for my mobo? Is there a formula to tell me?