Brion is far more scientific and knowledgable in tuning that I am, but on boats like this with a not particularly extreme spreader rake, I tune by first getting the head stay and back stay about right, then using the upper to center the truck. For this stage the lowers are slack. With swept spreaders, besides getting the truck centered I try to induce whatever mast bend I want. Only when that's done do I tension the lowers.
G'luck
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