I’m constructing a trellis this year, too, and your calculations match mine. Here’s some documents I found especially helpful.
1.Experiences-with-Support-Systems-for-the-Tall-Spindle-Apple-Planting-System.pdf (6.9 MB)
Building Trellis Systems.pdf (720.9 KB)
Mistakes to Avoid in Training Tall Spindle.pdf (5.6 MB)
Support Systems for High Density Orchards 2018.pdf (8.9 MB)
Trellis Systems.pdf (1.6 MB)