If the boat is just for you, and you are confident you know where your feet are, put the bulkhead back far enough to serve as the foot rest, and get rid of the footpegs. Rake it forward a bit. I paddled a glass boat like this once and it is total comfort; free to move your feet around and still always have instant bracing.

Since it would be bad to err on the short side, maybe put it a bit farther forward than necessary and customise it with a minicell pad. You could even get fancy and make a recessed area for the pump.

Another issue involving cockpit volume is the space beside the cockpit opening. A guy in Austrailia (Peter, Peter....can't remember his last name) essentially runs longitudinal bulkheads either side of the cockpit, between the fore and aft bulkheads. This makes a minimum volume cockpit to minimise pumping.