I just multiplied everything by 1.06 which gives 12 23/32" for the form spacing. The real problem, is what to do with the cockpit and recess. The forms will index an over size cockpit but the recess may want to remain the same size as the basic guillemot, have not made up my mind on that one yet. Of course this fitting is personal I hear, but this is my first kayak so I have no idea what that just right fit is. Pressing onward.