Steve said:

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.

That should work fine. Just put the seat in a good spot to get your thighs under the deck and also allow a bit of layback (depending on the type of roll you do..).
For a future build:
If the designer used software to develop the plan, the center of buoyancy location may be available. You could ask the designer.

You can find your own center of balance sitting on a short 2x8 in paddling position with a bit of pipe underneath.

If you find that the weight distribution is a bit 'off' it's pretty easy to toss a water bag in one end of the boat or the other on day paddles. On trips, the way you pack your gear will make more difference than a few inches change in seat position.

That said, your position in the boat does make a difference to the handling - the bigger (longer and heavier) the paddler, the more difference you'll see.
The Mariner sliding seat is based on that concept and it does work. If you get a chance to paddle one, you'll find it interesting, especially if there's a breeze.