@Jose, @Doug and @John,

Thank you for your answers. n the beginning I decided to fillow the Doug's advice, especially that I understand all the NCFZ rule.

1. I measured the full travel of the focuser tube using a caliper and it's 94.4 mm. In the meantime the EAF performed 25,992 steps.
2. I found out that 1 step/tick mens 3.631886734 µm, please correct me if I'm wrong.
3. I didn't find whether for the diffraction limit I should use the Dawes limit (120/D) or the Rayleigh limit (140/D) so I took an average value 130/D.
4. I solved the equation for the 102mm F/5 refractor and for everage seeing that is usually around 2 arcsec and my NCFZ is 42 µm.
5. I found on the 2nd page of the mentionet thread that the Step Size should be equal to 1/2 of the NCFZ. Should I understand that if 1 µm means 0.275338983 of the step/tick then the 1/2 of the NCFZ is 21 * 0.275338983 = 5.783512841
-> 6 steps ???

I cannot believe that so small movement may change something.

I really want to understand it, so Doug, please help me.

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