I like this idea.
When I start the night and before I start the sequence I perform a autofocus using the focus control panel. I typically like doing 3 or 4 autofocus back to back to see if I getting similar results. Typically I see changes from run to run of 100 to maybe 300 steps, (my focuser has 7,000 steps and I typically start out close to 3,500, at the mid point). so a variation of 100 or 300 steps between runs is telling me I might not be at the best focus.
Maybe in the focus control panel allow for a check box to enable or disable a fine step differential focus feature like Kurious_George outlined that would kick in after the autofocus run. I think this feature would improve on the current SGPro autotocus to make it even more robust.
Just my thoughts,
mahaffm