Hi Martin
I'm familiar with the reducer and I will throw a third backfocus distance into the mix.
Probably the easiest way is using the 90.4mm back focus distance. Look at the system diagram and locate this.
For instance TKA36581 (645RD Adapter) ---> CAA ((CCA-250) Part 6A) will put you at 90.4mm to go until you reach the focus point. I am assuming Takahashi are using the middle of their focuser travel for this calculation.
With all that being said, the reducer is in a fixed position, so you only need to put yourself into the +-10mm range of the focal point once your EFW, OAG, Tilt Device (oh don't forget that part
) is configured. Getting nice and close to that 90.4mm distance is great and all however its not critical, your focuser should have enough travel to pick up focus and keep it there all night.
Michael