When I was playing around trying to design a special purpose kayak (for duck hunting) I measured my CG in a paddling position by sitting on a plank on top of a dowel creating a fulcrum. I discovered my CG was approximately 12 inches forward from my back (with waders and wading boots on).

While reading a kayak review in Sea Kayaker magazine I noted with interest the location of their paddler's CG in a footnote that read "*Fixed "Paddler" weight has its center of gravity located 10" above the lowest part of the seat and 10" forward of the seat at back. The "cargo's" center of gravity coincides with the kayak's approximated center of gravity."

By definition the CG of the kayak and all of its contents has to be directly over the center of buoyancy. If it does not the bow will move up or down changing the location of the center of buoyancy so it is under the total CG.

If you already know the design center of buoyancy (CB) location and hopefully the empty kayak CG location you can calculate the paddler's CG location. The empty kayak CG should be infront of the center of buoyancy and the paddler's CG should be behind (kind of balancing around the CB). The distance from the CB to the empty kayak CG times the weight of the empty kayak should equal the distance from the CB to the paddler's CG times the weight of the paddler. (That's called a mass moment calculation).

To calculate the location the cockpit, without knowing the kayak CG and CB location, you must first calculate the location of the center of buoyancy of the kayak. Then calculate the weight and CG location of the kayak and cargo then add to that calculation the weight of the paddler moving him forward and aft until the total CG location matches the center of buoyancy. Now that you have got the paddler located you locate the cutout of the cockpit relative to the paddler. You'll need an engineering background to do all of the mass moment calculations or use a marine design program.

Sorry, the engineer in me kind of came out. Hope something in this rambling helps.