Sigma and Tamron are the only ones who actually instruct you to try to focus at what is essentially infinity. 50X is nowhere near infinity, even with a 600mm lens. I don't believe that the Dot-Tune guy was focusing at street signs two miles away.
I have, and use, FoCal, and with primes or zooms where I am adjusting just the ends of the focal length range, it works fine. However, I wonder if there is enough of the target in the image for the program to work when I am focusing at the shortest distance (7.2 ft) and the longest focal length with my Tamron 150-600. Likewise, if I am at the intermediate distance (22 yards) at 150mm, the target will be far to small for FoCal to properly do the analysis.
The developer of Dot Tune is not doing this at 2 miles. The way way I understood this was at infinity but as close as you can. He found the best results at infinity but I can't verify since this is passed on information. I know I asked and he responded at Fred Miranda a few years ago. It might have changed since.
I hit the same value with Dot Tune at infinity as I did using FoCal but I could not repeat using Dot Tune. That would be unfair to Dot Tune as I was outdoors and it was pretty windy when I tried using it again.
I still hate judging and prefer something to tell me how much to apply.