Proper headstay tension with furling unit

[Brion Toss]

Hello,
First, since it's the backstay that tensions the jibstay, put a gauge on the backstay. Then compensate, through trig, for the angle difference. If the two wires are different diameters (they should be, in almost every instance), compensate for that, too.
Second, what you really care about here is luff sag, not too much, not too little. The gauge is just something to get you in the neighborhood.
Third, playing with the jibstay also means playing with the mast shape; both must be happy.
Fair leads,
Brion Toss