The project is complete. I ended up using the Dieball tuning guide found on the DS Association home page because it provided more dimensions.
I measured 130" from the end of my transom to the base of the mast inside of the cuddy (only path to stretch a tape straight). This is line a on the drawing. The Dieball tuning guide says 132 +/- 1" for this distance. So, I went with 131" since my measurement was to the inside of the cuddy.
My masthead is 264.25" above the cuddy. This distance should be ~270.75" for a class-spec mast. This is line b on the drawing. I subtracted 3/4" from these values because that is how much my mast shortened when tensioned the rig.
The Dieball tuning guide gives a target of 300-303" for the masthead-to-transom distance with proper rig tension.
Using the class-spec measurements and the Dieball values for a and c, the angle between lines a and b (angle C on drawing) should be 90.68 degrees. I kept this angle constant and used my mast height to calculate the target masthead-to-transom distance for my mast. This came out to 295.7".
I then freed the forestay and used the jib halyard to hold the mast up while I tightened the backstays to reach this measurement. Once I had the proper rake, I discovered that I needed another 3.1" of forestay length (Or, I needed to move the forestay tang down the mast). So, I measured the distance from the base of the mast to the stemhead (x in drawing), calculated the angle between this line and the mast, and used my target forestay length to find how high up the mast I needed to place the tang to keep the angle constant. It needed to be only about an inch above the factory location.
Since the new forestay is longer, I will be sailing with more mast rake. Looking at the boat, the rake is noticeable. But, I still have adjustment in the forestay if I find the weather helm too much.
I have a week vacation on Umbagog Lake coming up, so I will post an update after sailing the boat in a variety of conditions.
This online calculator took all of the work out of calculating lengths and angles:
https://www.calculatorsoup.com/calculat ... osines.php