Commuting to work on my old school mountain bike I wear my Timbuk 2 medium pretty low, the main weight rests just above my belt. I adjust the chest strap so the bag hangs at an angle and centered on my spine.

That's the most comfortable for me.
My bag weight is around 15 lbs.
I am a cold weather commuter, 2 jackets, studded tires 2 hats, neck warmer. 5 degrees F. tonight on my ride in.
bill