The slope of the focus vs position is related to the inverse square of the focal ratio. Hence halving the focal ratio (f/7 to f/3.5) and you have a slope that is four times steeper, and FocusMax needs 4x less distance to do V-curves and run it's algorithm. From my experience with an ~f/7 scope, a f/3.5 would need about +/-1 mm of travel for FMax to function.
Analogously, the good old CFZ (critical focus zone) is also simply related to focal ratio. i.e. CFZ (µm) = 2.7 * f.r.^2
hence why f/3.5 scope are a PITA to focus, relatively.
So for Greg's scope the traditional CFZ is ~40µm
However I find the NCFZ to be more useful for specific situations since it takes into account the seeing at the site, the focal length and how much defocus you are willing to tolerate, see here:
http://www.goldastro.com/goldfocus/ncfz.php
for the formula.
Best,
EB