The seat position specified in the plans should be followed as closely as possible, IMO.
The designer should locate your butt at the right place in relation to the center of buoyancy of the hull.

That makes a ton of sense - first get your butt in the right place, then locate cockpit, bulkheads, and foot pegs (if any) relative to that location, with some adjustments for preferences, e.g., from what I understand some people prefer more distance between their backs and the cockpit coaming than others. Unfortunately, my plans don't specify seat location - so I have to work backwards from what is specified, i.e., the aft edge of the cockpit.

I appreciate all the feedback - very helpful.