The following are the measurements from the sheaves to the pin at the top of the headstay
upper sheave to pin 4 3/4"
lower sheave to pin 2 7/8

So to set headstay length first measure your spin halyard shackle and ball length which will vary from boat to boat

If the length is 3" and you are using the upper sheave distance from the pin to front of shackle is 1 3/4" add that to max head stay length of 35'10 1/2" for a tape measurement of 36' 1/4" inch

Don't for get to put a light line on the spin halyard so you can pull it down (tapes break)

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.

Bob - very good suggestion that doesn't require interpretation on where the pin is located at the attachment point on the mast. I was going to go up the mast on Rhapsody this week to measure a reference point with a tape. Now I only need to go up to change the bulb in the steaming light!

Charlie - would this be a consistent method to handle measurement at the NAs?

If this method shows promise vs. the alternatives, competitors could prep for measurement by releasing their headstay, have measurer mark the luff at the mast band, then return to measure the short distance from mark to stem once the forestay has been reattached.

I tested out an even simpler version of the above proposal. Take a jib halyard to the black band on the mast. Swing out to the headstay and mark. Measurement from this mark to bow should be 6'7". I tried it this weekend and estimate it is accurate to about 1/2 inch.

Great Plan Bob, That is how we measured the Forestay base length on the Ben 36.7 We adjust from there. I thought that should work for us.