Does you chair have arms that get in the way? What kind of guitar do you have? I could see an acoustic being difficult to get arranged. When I teach lessons I sit on a stool & have my strap set at the same length as when I stand. The end result is that I can have the same posture & position sitting or standing. Maybe you can adjust your guitar position in order to make it possible? Hope this helps!
Christopher my chair doesn't have arms that get in the way and i mostly practice on a standard nylon string, i also have a stedman pro electric. actually i find often times when i play that i have very bad posture, hunching over the guitar. i'll definately try your method on electric, adjust the strap till sitting and standing are the same. thanks a lot for your help. you guys are all so wonderfully supportive.
