Several years ago, one of our more "scientifically" inclined tow pilots rigged up a strain gauge at the tow hook and a laptop in the Pawnee cockpit to try to record loads and forces on the rope. I don't recall the actual numbers for my Pegasus, but the "scatter" in the data rendered the information virtually unusable. The "J" factor referenced above as the glider and tow plane encountered turbulence, reactions to control movements and out-of-position and/or uncoordinated flight made the load on the rope go from zero (slack line) to the weight of the glider. And it did it in such random steps that you couldn't make any sense of it. Maybe a more sophisticated system with more controlled conditions (i.e., smooth air) might reveal relevant results, but based on that one test, (as well as the data from several other tows with other gliders), I doubt any really usable information can be gleaned. The tow plane, tow rope and glider are in a very dynamic situation that is tough to quantify accurately. Good luck, as it would be interesting to see tests with better results.