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.
Yeah, monitor his body condition. If he's getting fat and pooping too much, he's obviously getting too much food. If he's losing weight and acting famished all the time, up the amount! Weren't you going to switch to raw a while back?
I agree with all. I had a dog who would eat all day...she was a little dog and NEVER gained weight as she was quite active. Now I feed my two dogs twice a day. Once when I get up (I get up at 4 a.m.) and later sometime after 4:00 p.m. They are fine w/that and yes, one will eat any time any place w/the other one hard to start her eating but then she does ok. I recently switched to Innova EVO and my one (eat anytime) dog gained 10#. So now I've cut back and I'm waiting to see results, if any. You just have to experiment as you go.