I consulted one of my favorite reference books, The Strip-Built Sea Kayak by Nick Schade. In Appendix 4 he suggests that the paddler's center of mass should coincide with the boat's center of buoyency. I'd suggest that if you deviate from this that you place the paddler's CM slightly behind the CB. That way you can adjust the seating position without recutting the cockpit opening. It's easier to move the paddler forward in the cockpit than back. A good approximation of a paddler's CM is their navel.