Not sure the critical focus zone is really the issue with auto focus. The import thing is to give SGP enough HFR data to establish an accurate minimum HFR. You want a statistically valid change in the HFR calculation between data points. As a guide line, you want your max HFR calculation (first one done after focuser is initially moved outward) to be around 4x your minimum HFR. Manually get a decent focus and then play with the auto focus settings, that is, number of focuser steps between data points, to get that 4x range. For example, let's say best focus produces an HFR reading of 1.0; you would adjust the number of focuser steps between data points so that the worst HFR calculation is around 4.0. Use 9 data points until you can determine that an accurate focus can be achieved with fewer.
One issue to keep in mind, however, is that the more out of focus you go, the dimmer the star images are. If you go too far out of focus on your initial focus frame, SGP may not detect enough stars to do a meaningful HFR calculation. Trial and error are your best friends.