Okay engineering genius-types, how do I figure out a good halyard size/strength for my mainsail?

Looks like I've got 5/16" (8mm?) class II double braid on there now..

But I have no idea what kind of loads the sail/halyard generate. This main is cleated at the base (not hooked at the top)

Mainsail Specs:
Luff 29'6" (9 M). I think this is measurement "P"
Foot 11'6" (3.5 M) "E"
Size 243 sf (22.5 sq.M)

Would load=(1/2E)*P work for a halyard? I think that's for end-sheeted mainsheet?

And if I read correctly, the working load should be 20% of breaking strength, so once I figure out the load(s) I need to multiply by 5 to get the break strength I need to buy, right?