If you are building from plans or a kit and have built the cockpit, you are limited in your seat position. Hopefully, the designer thought about trim when locating the cockpit.

I would start by putting the seat into the boat and sitting in it. Is it comfortable? does moving it back or forwards help the comfort? Do you hit the back of the combing if you lean back to roll or brace? Do you have foot room and a place to lock in you legs?

I use foam seats and a suspended backrest like in Nick's book and end up with my backrest 2-3" in front of the back of the cockpit, and the seat an inch in front of that.

If you already have the boat built, all talk about trim is academic, go for fit. If it doesn't track right add a fixed skeg.

--Eric