Position of the QuadTCC relative to the objective


I have some questions for the swarm intelligence aka ap-gto.groups.io:

I am installing a Nitecrawler electric focuser and rotator. The Nitecrawler replaces the Feathertouch focuser and the QuadTCC is screwed into the draw tube of the Nitecrawler. To adjust the position of the QuadTCC relativ to the objective I have a couple of distance rings.

To check if I found the correct distance, I used Astrometry.net, plate solved an image and calculated the focal length (57.3 / (image size / sensor size)). To my surprise the focal length I found was 600mm (Starfire 130 EDF GT) and identical with the Feathertouch/QuadTCC combination.

Is the method to calculate the FL correct? 
If yes, why is the FL 600mm and not 585mm?
Is there another way of finding the correct position for the QuadTCC?

I am getting a TEC 160FL next year and I plan to use a QuadTCC for the telescope as well. How critical is the exact position of the QuadTCC relative to the objective?

Clear skies,

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