Always go by weight when you can. It doesn't have to be exact to the point either... sometimes you cant get whats "perfect" for the animal, but u can get pretty close, it is just a guideline to help everyone..
Balls are tougher than what they seem to be, is definitely not good to have a hungry young snake, and at the same time if you over feed it for too long, it will go on a hunger strike, but at that age hunger strike is rare.