CFZ - Critical Focus Zone (micrometers)
θFWHM - total seeing (arc seconds)
τ - focus tolerance as a percentage of total seeing (unitless)
A - telescope aperture (millimeters)
f - effective imaging system f/ratio (unitless)
0.00225 - constant (micrometers per arc second per millimeter)
Given a user estimate of seeing, and their acceptable tolerance (both of which could be defaulted), and capturing system info (or having user provide) for aperture & f/ratio, the CFZ pops out. Step size is then just a fit of CFZ on the translation of motor counts/revolution and focuser travel (mm->microns) as discussed previously.