Alan,
When a triangular sail is sheeted in (as in going to weather) the loads on each of the three corners are just about equal; therefore: sheet load = halyard load = tack fitting load.
Safe working loads depends on application and your acceptance of risk. I use a safety factor of 200% for noncritical loads such as sails and 1000% for critical loads such as hoisting a person to the top of the mast. Using Bob's 3800-lb breaking strength for 3/16 Spectra, I would be "happy" using it for halyard loads up to 1900 lb and for hoisting a person up to 380-lb. (Although I hope I never have that much sail up in that much wind or have to hoist someone that big to the top of the mast.)
Another factor to consider is that for some of the wonder fibers the %elongation is nonlinear and becomes a lesser percentage as the load increases.
