Bit late.........The 48mm should be fairly accurate but you need to measure Center to center on the Slave cylinder........Green line, pic 2
Make sure you have all the components and use the measurements i have given as a starting point. The brake actuator that goes through the frame is the starting point, so the shaft is square to the MC as it sits on the brackets..........if that makes sense.
On the factory tubes/brackets, (the MC bolts onto), have been set into the frame a bit, not just welded to the top so make a grove about 5 to 7mm deep, then weld them in making sure your always square to any lines your working on
This thread from FB71 may help
https://www.xs650.com/threads/long-time-comin-82-heritage-restomod.51701/
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