Proper sizing main halyard

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I recently bought a Beneteau 381, and am in the processs of replacing the lines on board. I have formulas for a number of different line loads, but can't find one for halyard loads, this combined with the PO's selection of terrible lines throughout the boat doesn't engender a lot of confidence in replacing size for size.

I am hoping someone can recommend a load calculation formula, or a sizing recomendation for the boat.