Tim, you're right. It's the position of your butt, or more accurately your Center of Gravity, that determines cockpit placement. A reasonable approximation (in all three axes) of your CG while seated in a kayak is your navel. For most of us this is probably 12-14 inches in front of the backrest, which in turn is a couple of inches forward of the back of the cockpit.

I suspect that it's probably better to wind up with a cockpit placement that has you sitting a little to far back rather than too far forward. Think of the Aleutian baidarkas. They have very little curve to the sheer so the ends don't catch a lot of wind, but I'd expect they would tend towards leecocking and also that they would track well. Of course all that might change with a seal on the front deck.