You are soooo over thinking this.

Quit that.
Here's the deal. Feed him about half a pound a day, or a bit less. If he starts getting tubby, feed less. If he starts getting skinny, feed more. It's that simple. Food weight is not the only thing that will affect his overall weight. Some foods digest better than others, some go right through them. You can't control everything by micromanaging the perfect # of ounces of a meal.
Feed 80% meat, 10% bone, 10% organ, averaged over some period of time. Chicken backs are like 90% bone, so you need to balance those with meat or organ. I never really fed them because they aren't very nutritious. It's like us eating nothing but saltines.