Thanks to all. So it looks like the cockpit should be centered close to the designed centre of bouyancy or a little aft. About 12 to 14 inches of paddler, a couple if inches for the back support and there is the rear cockpit thwart at 18 inches from the centre of bouyancy.

Now that that is sorted out how would one determine the actual centre of bouyancy, give or take a little, of a framed and covevered hull? All I can think of is to fill it with water up to the planned displacement water line and the find the fore and aft balance section. Maybe that's why there called water lines?