Do not ask Tarzan how much he wants to eat. Being the smart alec he is he will say "a goodly portion." Sick, hungry, tired, busy, bored or angry, that's his answer.
I ask him how hungry he is. If he's "eh," (that's a real answer) I don't give him much.
If he's "starving" then I give him a lot.
And if he's hovering over the pot asking when dinner is, I just serve him all of it.
This is me and it's a proven system.