It really depends on the age of the musician. A young fellow will want a long strap so he can make huge exagerated motions with his strumming arm while jumping up and down. The long strap also helps with the suggestion that the guitar neck is actually an organ of some type.

An older musician may find he wants the guitar higher up so he can actually play music on it. Of course as many of us age, the guitar naturally moves higher on our bodies even without shortening the strap.

Seriously, though, mizzo has the right idea. You want the strap adjusted so the guitar feels the same sitting or standing. If you are right handed, sit in a straight chair with the waist of the guitar over your left leg and the neck point up somewhat in a comfortable position. Then adjust the strap so the guitar is in the same position when standing.

I like to place a strap button on the treble side of the heel of the neck so the strap must reach around the heel to go over the button. I keep the button away from the binding and away from the body but up in the meaty part of the heel. Then I use a soft leather strap with no metal parts and a length adjustment that's based on a narrower piece of leather snaking through selected slits in the wider part of the strap. The narrow part attaches to the button under the heel.

I don't recommend centering your strap button in the back of the heel because when your guitar neck is raised, the strap is pulling itself off of the button.

And always carefully drill a small guide hole before installing a strap button.

Also, not to cramp anyone's style, but… strumming shouldn't really involve a lot of shoulder motion. Unless you depend on the visual effect, you might want to talk to a teacher about strumming technique.

      - Mark