I cheated! Last year when it came off, I left the turnbuckle alone and just released the backstay.
Putting it back together, I basically assumed the mast top was in the right spot and not leaning back too far and tensioned up the backstay. Don's write-up on headstay tension I found a little vague, "feels firm when pulled on at shoulder height" - I "felt" the tension in the backstay instead - figuring it must be equal that of the
furling covered headstay.
What was interesting during the tuning of the shrouds was that I ended up with the starboard spreader rising up about 6" and had to pull it back down against a wire clamp that's on the stay. I wonder if there should be a clamp above the spreader too to keep it in place - or if there's a reason it's allowed to float (up only).
Bill