Headstay tension
Headstay tension is typically not measured directly because as you mention the furler kinda gets in the way.
Headstay tension is gauged by spar shape and backstay tension, as well as an educated shake of the foils.
Basically you want the headstay reasonably "tight" and still maintain a proper spar shape (again subjective). Spar shape and headstay tension are directly controlled by backstay tension.
And, of course, gauge tension is just the starting point, you need to sail the boat to truly tune it.
Bob
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