Yeah, bottom edge of mirror too low, for instance (stops too low). My caliper is accurate, based on measuring stuff like guitar wire, and other odd things of known factory accuracy. And on my home fabricated GG back, I used 3/16 hobby plywood from my RC plane building, and enough tape of various kinds to stand the GG off from the back panel of the camera where the film would be. The film is behind the gate opening by some distance. You can't just slap your GG on the camera back.
I was happy with my caliper accuracy when I measured the camera about 80 times and kept coming up with 71.40mm ± .02-.03mm most of the time.