Remember, Jimmy Page, Eddie, they slung low, but, they play the articulate stuff with the neck pointing upwards at average a 45 degree angle.....when they're laying back over some power chords, they flatten it out....
I agree with the sitting method. Sit, and play normally. There is your starting place. Adjust your strap, stand up, and you should feel fine.
Over time, lower it a bit. When the slop factor sets in, you know you're at your limit, but, try playing with the neck pointed up....maybe you can do it, maybe you cant.
Jazz guys dont care about looks, they play, and the guitar is where it plays best, at the expense of all else..
Here's a good one...
Is he slung high or low?

Ted Nugent, btw....
He's about center on the belt buckle...tho I'm sure he's not buckling his axe ....
Here's Eddie slung higher than normal.....
and ....Eddie's typical neck angle... though I can't find one of him in the older days....
Note Pagey's neck angle.... typical... like they're launching ICBM's...
