The actual step difference given in these examples is not normalized, ie. not meaningful without comparing it to the step increment. In our analysis we are working with the difference as a percentage of the step increment. This 30 or 60 steps might correspond to 300 to 600 steps on your rig. What is your step increment? In the case of my example, my step increment is 30, so the difference is 100% to 200% of the step increment, a really big difference. Definitely significantly worse focus.
Actually a quadratic curve (which is a parabola) rapidly approaches a straight line as you go up the curve. That is why the FocusMax papers refer to straight lines. The lack of symmetry that can occur with our curves is definitely a potential issue that will need to be dealt with. And we can easily deal with it. A straight line approach would have the exact same problem to deal with. One of the reasons we need a lot of sample log files. Can you send us some that are asymmetrical?
The big problem with this approach is demonstrated well by the current SGP implementation. Trying to work with two linear sides involves tricky and arbitrary choices of which points to put in each line. In every example, quadratic does a much better job.