How much should I feed my cat?

I have a 4 year old 12 pound cat. He is not overweight, just very long, tall, and muscular. I feed him Blue Buffalo Indoor Health. He cannot eat wet food as it upsets his sensitive stomach. I've been feeding him 2/3 cup. Is this enough or too much/too little?

if he not underweight or overweight it enough, can you see or feel his ribs? you don't want to see his ribs but want to be able to feel them. 

How about shoulder blades, how do they feel?

How active is he?

He has medium length fur so I couldnt see any of his bones even if they were protruding. I can feel his ribs but I have to apply very slight pressure to be able to, so I know theyre not visible. I can feel the tops of his shoulder blades, but there is fat around and in between them. He sleeps a lot as all cats do, but he has short spurts of energy

Oops. Posted before I was done. sleeps a lot but he has short spurts of energy throughout the day. He spends 1-3 hours running around and playing at night.

Usually the food has guidelines for feeding on the bag. The guideline may not be the perfect amount for your cat, but I would start there and adjust the amount based on what you see with your cat.