It's exactly Steve's mass principle, with some alterations based on experience, I've adjusted his formula but it's very accurate for longbows. My process is to get a stave roughed out, do lots of measurements and checks then weigh it and this forms my initial tillering. I never use long strings, as you can see from this I've fitted the knocks before even bending it once. I'm sure this is how bows would have been made 500+ years ago.