Finding the correct forestay tension starts on land, where we set the shroud tension and position the mast butt. On our J/24, we start with a default shroud setup, tightening the uppers to No. 20 on the Loos gauge, the lowers to No. 15, and leaving “two fingers” of sag on the forestay. (We say “two fingers” because, on the old-style Loos gauge, the tension is so loose that, when we pull to the black line, we can fit two fingers between the gauge and the forestay.) This is surprisingly loose: I can grab the forestay at about my shoulder level and swing it back and forth through about 1 foot to either side of center before it gets taut. Each class and sailmaker have a different method and set of numbers for measuring forestay tension, so read your tuning guide first, do your best to tune your boat, and then ask your sailmaker to jump on board for a sanity check.