If not willing to go above coaming height, then there is no right answer.
Either the seat will be too low or the back too short.
Measure any standard chair that is comfy for good dimensions. That's what you need but I will bet it is taller than what you are willing to build.
I hated my seats so much, I ripped both back to back seats out and live with folding chairs. Much more comfy and now I can have any seating arrangement I want.
If you insist on installed back to back, I would build the bottoms high enough to be comfy for off the floor height like a chair, then do short backs. If real uncomfortable, I would have seat backs made with 2 posts that could be raided like head rests in a car. Up when you need them down when not needed.