The first reef's luff cringle is 28" up from the boom and the leech cringle is 34" up from the boom which is about 21% of the sail area. After a little more math I found putting the second reef at 36" above the first reef will reduce sail by about 44%, which was my goal.
There were four reinforcement panels on the luff and four (larger ones) on the leech, and I took dimensions for all of them. I have a book named "The Sailmaker's Apprentice" and I reread everything having to do with sewing and reinforcement, etc., but in the end I decided not to do any of the heavy duty techniques suggested in the book and instead I mostly copied the existing reef points.
I made patterns of the panels and layed them out on the sailcloth in an efficient way, and then traced them onto the cloth. Less than 1/2 yard of 5 oz sailcloth was enough for this project, also ordered Defender's sewing kit for needles and twine and palm. I broke all the needles practicing because the needles were not sharp enough to get through multiple layers of sailcloth, and the twine was way to thick. But I found a whipping twine at West Marine that is only slightly larger then the existing sail thread, and found large sharp needles at a local fabric store where I also found "Liquid Stitch" seam glue, and an awl for poking holes in dense fabric. I used the Liquid Stitch glue for assembling the panels to each other and tacking the two reinforcement stacks onto the sail (the first (existing) reef used doublesided tape instead of glue). Removed 1.5 ft of bolt rope tabling and leech cord tabling to glue the reinforcements under the tablings. I drew pencil lines and hashes where the holes for the zig-zaged thread should go, and then poked the awl through the hundreds of hash marks.
Then the sewing began, 12 hours of it! I'm sure it could have been sewn in short order by my wife and her machine, but I wanted to sew it manually, for the experience. To make a z-z manually you make a crooked straight stitch so that half the zigs are on one side of the sail and half the zags on the other, then sew the same set of holes again but on opposite sides. Beginning and terminating a seam is a problem. If you tie a knot it is big and clunky and looks bad, and knots tend to untie with twine. Oversewing, where excess thread is tucked under adjacent stitches, works well and looks ok but you can't oversew a zig-zag very well. I also tried a dab of glue but the glue is big and clunky and attracts dirt. Also, the amount of thread to attach to the needle is a problem. If you don't have enough thread there's another termination to deal with. If you have too much the thread gets tangled and knotted and won't pull thru the hole. I ended up using doubled up thread about 2 to 3 ft long but using a finger to hold the thread taught enough not to tangle, and terminating with a single overhand knot and a tiny dab of glue on it, cutting the excess thread when the glue was nearly dry. The sailor's palm was worthless; I did most stitching with a needlenose pliers. When stitching was done I went over all the stitching with a hammer to flatten the seam as much as possible.
I used 1/2" brass grommets for the 2 reef point cringles. Had to sharpen the hole puncher to get through the 5 layers of sailcloth. Alternatively, I could have ordered stainless steel cringles and rented the the installation tools from sailrite.com but I already had the brass & tools on hand.
I liked doing this project so I'll probably do more like it- I'm thinking of cutting down and reinforcing an old jib to make it into a storm jib to balance a double reefed main, and maybe sewing some foam into the head of the mainsail.