Have you allowed for any adapters - if necessary ?
I'm not familiar with the FF - but if the backfocus or metal back distance specified for that FF is what you say, then so long as the distance from the back of the FF to the CCD is correct it should work.
You usually have a little bit of latitude on most FFs where an error really doesnt make much difference - that may up to a 1-2mm range
So if you are worried about being precise to the micron, you dont need to be, especially give the range of adjustment you mentioned.
Most FFs dont have any adjustment
Where it gets trickier is when you have filters or other optical elements in the optical train - where the optical distance and the physical distance may be different to each other
The AO8 actually does this too but if you have specified the SBIG specified distance that will be fine
Down load the trial version of CCD inspector. It as a routine that assesses the field curvature. You take a picture through you imager then analyse it and it shows the field curvature in you image. Make a correction take another image analys etc. it will let you know very quickly if you need to add spacers or adjust the Focal reducer.
