There really are no "rules" per se. Every dog processes it's food differently, and what may be sufficient for one dog will not work for another. It's a lot of trial and error. I would start with the guidelines on the bag and adjust from there. If he's gaining too much weight, cut the portion down. If he's losing, up his food.
My senior Siberian (56lb) eats 1.5 cups a day of an excellent grainless food, sometimes less depending on his mood. When he was on crappy Science Diet (before I entered the picture LOL) he was eating over 3 cups a day.