Bill -
If this is your first boat, place the seat exactly as Pygmy suggests, then work from there. You need a baseline. As I seem to recall from building my Tern 17 and 14, Pygmy suggests placing the back of the seat no more than 4" from the back of the coaming. Different wind strength, paddling direction, and wave conditions will all effect how much the boat weather-cocks. You DON'T want it to lee cock, it will simply blow away from you if you fall in. The only point of paddling I remember struggling with in the Tern was the direction most paddlers struggle with, paddling with a quartering sea with the wind on the aft quarter.

Jim