I tried a new thing: actually following someone else's instructions.
Since I followed Rob Mack's instructions for building a box beam, I figured I'd also follow his instructions for setting up forms. They are pretty good. I set up the top alignment string with two strong wooden posts screwed into the box beam, and aligned it with the bottom string, using a plumb bob, just like the illustration.
In the past, I had suspended a top string, and snapped a chalk line on the beam to mark center. I aligned the forms to the top string, and used a spirit level to level them. This time, I have two parallel strings, longer than the entire kayak.
Setting up the first form, #9. The mounting brackets are dadoed to allow the string on the box beam to pass through. A strip of wood is stapled to the form along the center line, leaving room for the thickness of the string. When I brought the stick to the strings, and checked one of the horizontal lines, it was level!
Not, however, that I got it right the first time. When I got to setting up the bow form, I discovered that I had miscalculated the height of the string, and the tip of the form ran into the box beam. I managed to break the bottom string, then had to relocate the top string to a higher position.
The bow form in place. I cut a 3/4" slot in Form #2, and attached it to the back of the form, so as not to cover the lines. I also made the small fork-shaped bracket next to form #1, to adjust the bow form across the box beam, and keep form #2 at a right angle to the beam. Everything seems correct with a spirit level, and the tip of the form is lined up on the bottom string.
I did the same with the stern form. I can see how I blew it the last time. Everything straight and level is a very fine point.
I tried some test strips, and everything came out even & fair, except for form # 10.5, which had to be moved several inches.