Apparently there are two versions of that focal reducer which have two vastly different back focus requirements.
https://stargazerslounge.com/topic/2...-f63-question/
As for calculations.
BF = Back Focus requirement of focal reducer
FIL = Any filter you may have in the optical path
CCD = The back focus consumed by your camera
Spacer = Spacers required
BF+1/3FIL-CCD= Spacer
I was calculating this morning for a Nikon lens with my setup. I'm using Astrodon filters which are 3mm thick so a third of that is 1mm.
46.5(Nikon Lens)+1(filter)-10(M54 Nikon F-Mount)-4(M54 F Thread)-15(Filter Wheel)-10.5(CCD)= 8mm
This means I need an 8mm spacer to get the desired back focus.