I think many of us (especially those with HDPE arches) used the Johnson video as the starting point and I think he gave some approximations based on the size of his arches and yes it does involve having a couple of inches of "play" in each end. I got some short bungie cords and tied them off in the "base" and hooked into a 3d printed piece I got from someone on the forum so my lighted section is about 4 inches up the tube.
https://www.youtube.com/watch?v=4pjHDMx92TI