Geoff
This link is an interesting read.
http://www.morrell.ws/index.php?opti...tice&Itemid=42
It seems to imply that that one should simply adjust the spacing until the least coma is achieved at the edges rather than using a fixed spacing as the focal length changes every time you move the rear mirror on an SCT and the reducer only works well at a specific focal length.
Cheers
Stephen