You really want to get your Max HFR to be around 2-3x your min HFR. For instance using your min of 0.8 you want to shoot for a max of 1.6-2.4 as the top of the auto focus graph.
Essentially adjust the step size to get there. Focus with refractors is pretty linear up to a point so this should be fairly easy to find with a little math if you care to do it, or with some trial and error. If you can post your SGP log from some AF runs we can probably get you pretty close. I wouldn't concern yourself too much with the step size...but I do agree it seems higher than most, but that can be because of a handful of different factors.
Also 5-7 data points is generally sufficient. However 9 will allow you to "push though" for poorer seeing.
Thanks,
Jared