Tony, et al,
Reviewing available info on the web, I can't honestly say that any manufacture of correctors/ reducers etc. categorically say that an "absolute" spacing distance (as distinct from an effective optical distance) is required by these products.
I therefore must conclude that the general solution applies - add approx 1/3 the filter thickness to maintain focus.
(One reason for the "absolute" comment was the subtle change in plate scale brought about by the increased optical distance....)
Sorry for any confusion!
