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Graham,
You could use a reticule eyepiece where you know exactly where the eyepiece focus lies, or even project the moons image onto a translucent sheet behind the focuser....
At this "prime focus" position, there should be at least 23mm inward travel available.
( BTW I found the DMK which has a quoted 17.5mm backfocus [for the C lenses etc] only had 12.5mm when you remove the adaptor ring and use a std 1.25" nosepiece)
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