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 CB is under the total CG. So if your plans show the location of the design CG or CB your back should be 10" behind that location. Add at least 3" to allow for leaning back while rolling before locating the back of the cockpit.

However I found in my Yare I needed to move the seat back from the location on the plans to avoid weather vaning so I am enlarging my coaming (and cockpit opening) 2-1/2" to the rear (to avoid adding ballast in the aft compartment).

A previous related post I made on leecocking might help:
If you picture yourself seated in your kayak crosswise to the wind, there is an area presented to the wind that creates drag, a force applied by the wind. That drag force pushes you and the kayak sideways. The sideways movement of the hull through the water creates a lateral resistance force on the side of the hull pushing opposite in direction to the wind. Both of these forces can be represented as point forces acting at their respective centroids. If the centroids line up vertically with each other your kayak will not tend to turn (yaw). Now then, if your kayak is "leecocking" the wind force is in front of the lateral hull resistance force and a moment (rotation force) is created that will rotate the bow of the kayak downwind. So how can you reduce this leecocking tendency? Two ways. 1- Put more front keel in the water and/or less aft keel which will move the lateral center of resistance of the water on the hull forward. 2- Put more wind area (deck) aft and/or less forward which will move the wind drag aft. Adding ballast in front or moving the paddler forward will lower the front keel, raise the aft keel, lower the front deck and raise the aft deck, all help move the two centroids toward each other. Putting a sail on the rear deck (rudder in the up position) will help. Putting the rudder in the water or adding a skeg will make it worse (unless you have enough forward motion to create large enough lateral hydraulic forces, just don't stop paddling). Adding or removing keel or deck are not easy to do on a finished kayak. So I would first move the paddler forward and then add forward ballast if additional trim is needed.

Sea conditions and forward motion of the kayak also affect weather vaning or lee cocking of a kayak so being able to move the seat forward or back to trim the boat I think is a nice feature. My foam seat is attached with Velcro and the seat back adjusts with buckles in the supporting webbing so when I finish my coaming enlargement I hope I have enough seat movement to trim the kayak and still be able to lean back. Time will tell.

Hope this at least provides some amusement.