I've mentioned previously in the forum the difficulty of accurately and consistently measuring the J/30 forestay. Boats have a variety of halyard shackles with or without stopper balls. Binoculars are often used to eyeball the distance between the end of the measuring tape on the halyard and the forestay eye. Some measurers or owners choose to send crew up the mast in a bosun's chair to hold the tape. But in either case it is still difficult to measure the length of this forestay catenary swaying in the breeze.
Reading the North tuning guide suggests a simpler process. A jib halyard is attached at the bow and the forestay turnbuckle loosened enough to release the pin at the stem. Now the forestay can be held, vertical and straight, alongside the front of the mast aided by gravity. The distance to the upper edge of the lower black band at the mast is marked on the forestay.
Using the specs in the class rules, the Forestay Height (height of fore triangle from deck sheer line to the highest point of jib attachment) is 10414 mm less the distance between the Upper edge of the lower mainsail band on mast from molded mast step (2698 mm) and the mast step to sheer line is (1210 mm):
10414 - (2698 - 1210) = 8926 mm
Using the max forestay length 10935 and subtracting the distance from the mark gives you the short distance from the mark to the measurement point at the bow:
10935 - 8926 = 2009 or 6' 7".
Once the head stay is reattached, the distance from the mark to the bow can be quickly and accurately measured.