How much should I be feeding my adult cat?

I have a male cat, approximately three years old. He looks like he's a good weight, and the vet has never said he's not. He's fairly active as cats go I guess, allowed to come in and out of the house as he pleases during the day and locked in at night. But the problem is he's always whining at me, presumably for food and I can't work out if he's just being a pig or if he's actually hungry. I feed him a quarter of a can of meat a day, and about the same of dry biscuits which gets topped up when he finishes them. Does this sound about right? Sometimes he whines when there are still biscuits left, so he can't be all that hungry. What do you think?