Sit in a chair, canter slightly forward with arms out. i.e. in a riding position.
Now raise and lower your legs. See how the movement in your hips has a slight affect on the angle you carry your back/bollox?
This will raise or lower your head an inch or two depending on your height. So the key things are:
- seat height (I use the lower setting despite being tall)
- physical height (I'm 6ft 2")
- riding stance (seat, bars, reach, pegs, in-step ratio determined)
i.e. In-step - because the longer the leg the more upright you'll sit ergo more air you'll hit.
Sorry I clearly should have been more pedantic and given you my arm length too - because this is a fundamental factor in determining torso angle in relation to riding stance as observed above.
If you disagree, like I care?
jerkit