It is much easier to use the string method for measurement if you are on the factory floor at the Moyes factory, but good luck doing this in a tent in the landing zone at Laragne. One cow pie here and another one there. Also, try this on launch.

Measuring sprogs inside the sail? I don't see how that can work. You don't want the sail holding up the sprogs. You want the sprogs holding up the sail. You want the cable holding up the sprog, so it must be free to do so.