I've been slacklining since about April of 2006, and I've been wanting to get into highlining ever since then. Hopefully I'll have the time and the opportunity in the near future. While pondering the physics involved in highline rigging, I started to work out some equations for the forces that a slackline places on its anchors, first in simple cases, then in more complicated ones. One thing led to another, and before I knew it, I had written a 22-page paper on the subject. It's probably wildly inaccurate, but I can at least be pretty certain that the mathematics is correct. Some of the formulas that I derived in the end were surprisingly elegant (that's the only reason that I went so far with it), and a few of the conclusions that I came to were a bit surprising.
DISCLAIMER: I do not claim that the results of this paper or this calculator are accurate or relevant to any real-world scenario. They are based on an assumption that may very well be flagrantly incorrect. If you use these calculations for anything, you do so at your own risk.