Backfocus calculation

Hi Ewen, it looks like you're getting confused between all the different measurements.

For the backfocus, the distance from the camera sensor to the front flange of your Fuji is 17.7mm and the thickness of your T-adaptor is 37.3mm. This adds up to the standard 55mm. With a standard Canon DSLR, the sensor is recessed much further back to make room for the flip mirror, this is why a Canon T-adaptor is so much thinner, but the optical length is the same 55mm. Your Flat73 requires an additional 11.4mm which you can set with the built in scale, or more accurately with callipers. The total backfocus of this system is therefore 66.4mm, which will be the same regardless of which camera you use.

The 48mm refers to the thread size M48 which will screw onto your Flat73 reducer and the 45mm imaging circle means it should cover the 30mm diagonal size of the APS size sensor nicely with minimal vignetting.

Hope this makes sense, have fun with your new scope and camera!

CS, Tony.