If it looks like a blood python it's too fat. If it looks like a corn snake it's too skinny.
I think somewhere I read that 500 grams / foot is a great measuring tool for ADULTS to determine if they're good body condition to breed. I don't know if there can ever be a rule like that for babies and sub-adults since the ratio changes a lot as they go through growth spurts. Think of the way humans change body condition as they shoot up in height then fill out in weight, etc. It's just too variable.