Hi John, non-professional sites typically have seeing in the 2.5 to 3.0 (or worse) category. Folks love to claim they have sub 1 arcsecond seeing in their backyards, but well....you know how that goes. Anyway, I'd suggest defaulting to 3 and then users could change it if they want. You could also default tolerance to 10% (which should be fine).
Remember that only very fast telescopes (f2 - f5) will be particularly sensitive to the values. Those with f/7 to f/10 systems have such large CFZs that it's not going to matter much. About the thread pitch, I was being technically precise, but not implying that folks need to locate that info. The vast majority of people will simply run some reasonably large number of motor counts and measure the millimeters of focuser travel. If you allow entry of those numbers in the GUI (measured mm travel and motor steps for that travel) the alg could solve for microns/step internally to avoid that confusion for users. Measuring the focus travel is the only "hard" part; the motor spec will give counts/revolution via a simple google lookup. . Cheers Doug