What's the ideal place against my back to locate my new Timbuk2 messenger bag?

Ok, so I've had my new Timbuk2 laptop messenger bag for about 24 hours now, and it is already THE GREATEST BAG I HAVE EVER OWNED. All black with a red bottom (boot?) It rocks. Nice work, Timbuk2!

But here's my question -- I've been fiddling with the length of the strap, trying to figure out how and where to best position the bag when I'm walking around the city. In the past, I've avoided messenger bags because the single over-the-back approach caused me some serious back pain if I wore it too long -- but I can't give this bag up!

So what's ideal for minimizing back pain -- any idea? Is there a proper place to locate it when carrying it, or a way to gauge when the strap is the right length? I don't really ride my bike with the bag much, it's just walking, if that helps any.

Your assisance muuuuch appreciated; I don't want to have to give the bag up.