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barry,
You're mostly correct in your assumptions...
The focal reducer spacing is driven by the need to present "acceptable" images...some designs are very sensitive to the spacing. Incorrect spacing can cause significant aberrations.
For the camera - the back focus is a physically fixed distance from the front flange to the plane of the CCD. Obviously different cameras have different distances.
The "standard" for a DSLR is 44mm, 55mm when a standard T2 adaptor is fitted.
Hope this helps.
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