Dave,
There seem to be two possible issues here:
- the 'correct' position for the reducer with respect to the objective and eyepiece or CCD for a given reduction
- the matching of optical characteristics between the reducer and a particular scope.
I have no idea about the second, but the first is simple enough mathematically as long as you are given the focal length of the reducer, which for some reason I don't understand the suppliers rarely provide.
Which one were you asking about?