The assumption is that the kayak is designed to float flat on the water and dip forward or backward.

On boats I designed, I sit in the boat with an observer watching - no seat, sitting on the wood. I move a tiny bit forward and backward and get the opinions on where I am at the most neutral flat floating position. Then I put tape behind my butt to mark the spot. Somebody who is tall with fat legs will actually have a different position. I'm tall but thin. As far as the backband distance from the seat, once again it depends on your anatomy. Basically that's about comfort.

There's no such thing as the perfect boat. You might find that your boat paddles best in chop with weight back or on flat water totally flat? The longer the boat, the less variation you sill get in seat position/balance. Before anyone goes nuts, remember, commercially made kayaks place the seat in the most neutral position and everyone seems to accept it.

One thing I find that makes an enormous difference is seat height. Try to get the back of your butt down as far as you can. You will not loose seat comfort doing that and gain very noticeable stability with even 1/4" difference.