I'm trying to play with the h/w I have in hand but i have still 1mm BF difference between the two consideration, basically due to the diffrence of BF contribution between EFW (20mmm) and 21 mm extension. Am I wrong somewhere?
I don't have an answer, but an associated question and that is the minimum chip to filter distance that can be achieved with the EFW 8x1.25" fitted to the ASI294 - to then estimate vignetting with 3mm thick 1.25" filters.
The best solution of 55mm back focus length for your reference https://astronomy-imaging-camera.com/tutorials/best-back-focus-length-solutions-55mm.html
