Thanks for the replies, now a related question. Do you apply (install?) it with the mast centered in the boat, but without final rigging tension, or do you completely tune the mast, then apply it?
If it matters, I have a single spreader masthead rig with inline and forward lowers, but no aft lowers. The mast is fairly bendy, in fact I have a little trouble getting as much headstay tension as I would like before bending the mast more than I want.