Interestingly enough, the issue of shroud angle and strain is closely related to the issue of righting moment. (I recently posted some rough data about Potter stability on the Trailer Sailor Bulletin Board. )
You might think you'd have to calculate the force vectors generated by the sail under various wind velocities in order to know how strong you have to make the shrouds.
But in fact, the sailplan is not important at all. The rig has to resist only as much force as it would take to capsize the boat. The shrouds transmit the lateral loads to the hull. The hull resists heeling. The righting moment generated by hull form and ballast defines how much resistance the shrouds are subjected to.
So to figure out how strong to make the shrouds, you just pull the boat sideways, with a line attached to the top of the mast (with a scale on the line), do a few calculations to get from foot-pounds to pounds, check the scantling rules for the safety margins, and, voila!, you know how strong your wire shrouds have to be.
Pretty elegant, huh?
Best,
Judy B