Hi-

My first skin on frame was made from Bob Boucher's video, and he had you put the backrest 12.5 inches behind the center of the boat. The boat weathercocks a lot, which is supposed to be normal for a traditional Greenland.
My next boat I moved the backrest 20 inches behind the center of the boat, and it handles much better in wind and still hasn't any lee helm.
Possibly find some boat that perform the way you want it to, and measure to see where the seat position is, and transfer this to your boat? When I made my 2nd, i took some exploratory measurements from a Betsy Bay Valkyrie to get an idea of where to place things.

Good Luck-

Jack Gilman