Today I have had to educate myself about backfocus as i want to enjoy the frustrations of Astrophotography and have built my kit up to the point where i am almost ready to take my first photos. So back focus is the distance the sensor in the camera has to be away from the rear of the flattener in order to achieve pin-point focus / flattening. I can only talk about my setup as being new its the only one i have any experience with, but i am sure the lessons translate to other brands and setups.
I have William Optics 91 FLT and the 6A III 0.8x fattener / reducer also by Williams Optics, currently fitted to my first camera; a ZWO ASI 585.
If you happen to use the same set up as me, or, if you have a similar scope that has an extension tube fitted as standard on the scope (the one on the WO 91 FLT is 40mm wide and is in front of the rotary lock) you may need to remove the extension as with the WO 91 FLT in order to achieve focus with the flattener reducer fitted.
The ASI 585 needs 55mm of distance (back focus) between the camera sensor and the back of the flattner reducer, the scope also demands that there is 5.4mm of adjustment on the WO 6AIII, so all in all for me with this setup it works out like this:
Camera has 17mm of backfocus built in to the body as standard. = 17mm
Thread adaptor + extension tube is a total of 31mm. = 48mm so far
6AIII is adjustable, so an adjustment of 5.4mm as is needed by the scope + an additional 7mm to make up the difference and take the total to 60.4mm.
I hope this helps.