I have to replace my forestay foil (blew up Saturday in 20+). Once the new one is on, how do I assure that my forestay length is within class specs? How do I measure with the using a long tape and the forestay attached?

If you can, before installation measure from pin to pin with the turnbuckle at max class length. Then measure to the top of screw for the lower turnbuckle. In the future you can just adjust to the last 12-18 inches from that point to the deck. It's late, so I hope this makes sense.