Back Focus measure from???

[vindubs]

I have a Williams Optics Z73II scope with a Z73R 0.8 Reducer/Flatener.  ZWO 1600mm Pro requires 56mm Back focus but where to I measure this from?? 

[Whirlwind]

For WO products you generally measure from the base of thread (when screwed all the way on) to the location of the sensor.  It's not the camera that needs the backfocus but the flattener.  Most cameras/filterwheel/OAG set ups are designed with this length because it matches standard DSLR distances.  The small amount of extra scope for adjustment is to allow filters to be included (which increase the optical distance between the reducer and sensor).