Sorting out backfocus

[Dgwilde]

Hi folks,
This is my first post to the group. I just ordered my first ccd, an Atik One 6.0 and I have questions about backfocus. I have been currently imaging with a Canon DSLR and Astrotech AT2FF field flattener on my f7.5 952mm fl refractor. I never worried about backfocus before. I just screw the field flattener into the T-ring and everything worked. The field flattener requires a 57mm backfocus +- 4mm from the shoulder and the Canon had a 55mm backfocus.
Reading the specs for the Atik One 6.0, it says it has a 24mm or 28 mm backfocus depending on where you're measuring from. I don't have the camera yet so I can't see the diagram to sort out the measurement.  But, since the field flattener needs 57mm, does that mean I have to add a 33mm or 29 mm spacer between the field flattener and the mounting point on the camera? Which spacer 33 or 29mm? How does the nosepiece that comes with the camera figure in to it. Is that just used to mount the camera to the focuser if there is no field flattener involved?
Also, with my DSLR I had to add a 2" extension to the focuser tube to reach focus.  Will this be similar with the ccd?  Can someone advise me on how I would calculate the length of extension I would need.
I'm sorry my questions are so basic, but reading everything about back focus with these long imaging trains makes my head spin.  I don't intend to add anything else but the flattener into the mix at this point, but I still need to have an understanding of what I'm doing.

Thanks for any advise you can provide,
Dennis Wilde