I use (per Brion Toss) 20% of the Ultimate Breaking Strength (of the wire) for the upper shrouds, 15% for the intermediate and 10% for the lowers, with the foward lowers at maybe 11% or 12%.
This is more or less what I am following. In a video I have of Brion's he specified 15-20% for uppers, so 20% would be on the high end.
All of this is good for an initial tune at the dock. It's necessary to take the boat out, close hauled, in enough wind (14 or 15 knots maybe?), to load up the windward shrouds and unload the leeward ones. The lee shrouds should be loose, so that you can wiggle them around a bit, but not floppy loose. Floppy loose = shock loads = baaaaaaaaaaad.
I tuned my uppers to 15% at the dock recently, but last week finally got around to taking out the boat to look at them on my sail back from Catalina. In about 13 or 14 knots the lee shrouds seemed just a tad tighter than I wanted--maybe. So when I got back to the dock I backed them off very slightly on the Loos gauge. We'll see how they look next time I get a chance to go out in a suitable amount of wind. They were already pretty close, so this is just a process of dialing things in.
