I am not an expert sail designer. I have consulted heavily with Michael Storer of Storer Wooden Boat Plans and designer of the Oz PDR. I have interpreted his patent attempts to educate me into the following design.

The first consideration is how much sail material to add to take the bending of the spars into account. When hung properly, the yard will be under stress and will bend.

I have estimated the spar bend at about 2 1/2" (64mm) when rigged. This is a guess, but it is my guess, so I am sticking to it.