A subject that is talked about a lot on astronomy forums, is the correct spacing for focal reducers on SCT telescopes, from imaging plane to reducer.
This is because on these type of scopes focusing is achieved by moving the primary mirror, which in turn will alter the final F ratio.
Here is a chart sent to me from a guy on the Stargazers Lounge forum, very useful. Just input the correct Focal Length for your own reducer in the yellow box and will calculate the F ratios for given spacings on SCT scopes.
https://dl.dropbox.com/u/1071867/Focal%20reducer%20calculator.xls