I'm thinking about building a prototype backpack that's custom fit to me and will test out an idea or two, but I have a few questions about proper design that I hoped someone could answer.

1) How far below the waist should the pack hang?
2) Is my torso measurement (from waist to the prominent neck bone) the ideal exact distance between the top and bottom strap attachment points, or is there an allowance here or there for something I'm not thinking of?
3) Any tips for deriving a shoulder strap shape that fits my body best? My idea is to draw on myself with chalk and transfer that to fabric, but I'm not sure exactly where it should lie against me.