Fishman,
I hope I am reading your question right regarding spiling the outside plank? If you were to mark back, say 2" (with a compass or dividers), from the finished plank edge. Do this along the edge from the chine to the sheer, you could then dry fit the next plank over (still showing your marks),then simply by using the marks measure back onto the dry fitted plank the 2". Take off the plank, join the marks with a batten. Would this would give you the curve you would be looking for?
Trev
Great job so far!