Skywatcher Esprit 100 back focus with ASI294MC camera

[GrandPop]

Hi All, some advice sought.

I have on order a Skywatcher Esprit 100 and a ASI294MC camera, I intend to use a Astronomik CLS filter as well. How can I determine the spacers required in order to get correct focus.

the retailer suggests a “suck it and see” approach but it must be possible to calculate this! 
 

[PeterCPC]

From ZWO web site:

 

Edited October 17, 2020 by PeterCPC