The thickness of the wood

I'm putting thwarts (seats) in the boat.  There are few straight angles or lines on a boat.  All the thwarts are trapezoid shaped and cut to a bevel to fit the hull.  This probably isn't very complicated for boatbuilders, but for people with only house building skills, and few enough of them, it can be demanding.

I was in school for 20 years counting kindergarten and post-graduate work.  How is it in all that time nobody ever taught me to allow for the thickness of the wood?