Michael,
I honestly don't think there is any general confusion...
Adding, say a 3mm filter between a corrector/reducer etc. will add approx 1mm to the required spacing.
The issue, which I think is confusing, is the IMPACT of the change to the performance of the corrector/reducer etc.
Determining the position of the "new" focus is easy, but I can't find any definitive data on the impact of this on the performance of the correctors/reducers etc.
