Thanks for the reply, yes thats the CC I have on order, unfortunately I don't have the room for a variable spacer.
My camera QHY183M (17.5mm BF) plus rotator (12.5mm) plus Filter wheel (14mm having removed the 2" receptacle so using M54 threads both sides) plus the M54/M58 adapter (2mm) to go between the CC and the Wheel (not shown), gives me slack of 9mm (at 55mm BF from the CC) for the M54/M42 adapter that I haven't ordered yet to go between the rotator and wheel.
If the BF on the CC needs to be reduced from the stated 55mm for my FL, then I will reduce the 9mm adapter accordingly (I'm getting this made so I can order any length I like as long as it's over 1mm)
Once it's all screwed together there won't be much I can tweak, apart from adding spacing shim washers into one or more of the threads, but that can only add small amounts.
It's not easy to tell from your pic but it looks to me like you're a little under 55mm.