Assuming the seat puts me somewhere in the middle of the cockpit

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.

Ideally, the cockpit should then be located in relation to the seat position.
You want:
-your thighs to be under the thigh 'hooks' or the 'keyhole' cockpit sides so you can brace in the boat and roll it
-some space between your back when sitting upright to paddle (i.e. not slouching) so that you can lean back (lay back) for rolling. A reasonable space would be about a fist width, or a fist hitchhiking (thumb out) distance.
-enough room so you can get your feet in and out of the boat when your butt is in the seat. assuming you are using a 'standard' size cockpit.