That's not a good rule of thumb. Put out a pile of food the same size as your tortoise. If he eats it gone you didn't give him enough. If, at the end of the day there's still a lot left, you gave him too much.Thing is the quantity I read that he should eat an amount about the size of his head but I give him that and he stares at me like I want more he eats 3x the size of his head..