@madgrizzle: So part of your problem here is that the “Distance Between Sled Mounting Points” is meant to be for calibrating Quadrilateral kinematics. In switching it to Triangular, that particular measurement should be obsolete. I myself have noticed that changing that value does have an effect on the calibration of the machine, but I haven’t tested it thoroughly enough to actually know if that is strictly because of it or not. I have been doing A LOT of calibration work on my machine so some of the tests are blurring together. If that is the case, it’s a bug in Ground Control.
I think right now I have somewhere is the realm of 200 set for my distance between mounting points. Having a zero in that field may cause Ground Control to have some trouble with it, so regardless it may be a good idea to put a positive value there.
I had some trouble getting the last calibration step to work. Ultimately, I used the old calibration method I used when I first got my Maslow set up and got pretty reliable results. In Maslow settings and advanced settings, there are a couple of values that can be tweaked to dial in the machine. Triangular kinematics should use two values, the rotation radius and the distance between motors. I then cut 100mm x 100mm squares into the face of a test piece until the machine was within tolerance, ±0.5mm.
Maslow Settings:
Advanced Settings:
Disclaimer: These values are from my dummy Ground Control and are not the actual values that I use on my machine!
The rotation radius depends on which linkage kit you have. DLang’s top mount or Logan’s 45 degree kit threads should have some mention as to what a good starting point is for this number. After that, you probably could use the automatic calibration process to help dial in the machine.
Good luck! 
Let us know how it goes and if there is something you need clarified.