In my sketch, item C is a custom adapter that I purchased from Precise Parts. On the Precise Parts web site I specified the A-P 92 Stowaway flattener (92FF) on the scope side, and on the camera side I specified the SBIG FW8S-STXL Filter Wheel with Dovetail noseplate. The dimension "C" on the sketch is the effective length of the adapter. The determination of this dimension is discussed below.
If you look on the sketch you will see dimension "X". This distance (64.1 mm or 2.254") is the specified backfocus distance from the rear face of the 92FF to the CCD sensor surface. This dimension comes from the specifications for the 92FF from Astro-Physics.
This nominal backfocus distance is the sum of dimensions A + B + C + D where:
I ordered the spacer rings from Precise Parts because Astro-Physics does not offer them - Yet. I believe I saw a post where they plan to do so soon. These spacer rings have to fit inside the custom adapter and rest against the shoulder. They were not cheap, and I would suggest asking Astro-Physics when they will be available before you go to Precise Parts. If you go the Precise Parts route, you will specify a washer/shim on the scope side, and on the camera side you must specify the A-P 92FF/92TCC Canon EF camera adapter. The effective length is 1.0 mm.
I hope this helps