I've always worked from whatever the optical back focus requirement is from one point +1.
I have 53mm of working distance from my M68 to M48 adapter, the calculation would be:
54-23.8(current spacer I have)-15(filter wheel)-10.5(QHY163M)= 4.7mm
So if I want to put my QHY163M on my Sky Rover I need to add another 4.7mm of spacing which I will do but need some new screws
Unless someone tells me I'm doing it wrong!