For me, the linear algorithm for autofocusing was a great addition in Ekos (thanks for that!), making it way more reliable with the focuser I have. In my case, the main problem with polynomial (or iterative) is that it tends to take few samples, so the computed solution can be quite biased. Also, though my focuser implements backlash compensation, the back and forth movements won't help either. The linear algorithm does a more comprehensive sampling (almost without going back and forth) so its computed solution after the first pass is quite spot on.
The current implementation added a more persistant behaviour that in my case means that the complete routine takes a lot more (full field work best for me to better estimate the HFR in my noisy CCD, but downloads are slow, even with bin2x2).
Would you (@ Hy Murveit) consider to add a "single pass" option to the linear algorithm? -- meaning "jump to the computed solution after first pass" (with compensation) if curve fitting is good enough.
Another question, regarding the initial movement (outward+inward): it seems to implement a backlash compensation of 5*initial_step_size, is that so? Could this also be configurable?