Figuring back-focus for a C11 Edge HD

[Edward Orlando Wiley]

I have a 0.7X reducer on my C11 Edge HD. In addition I have an Optec focuser, EFW2 Filter Wheel and my Atik 4000. Can anyone confirm if I have calculated the back-focus starting from the rear flange of the reducer?

Optec focuser = 88.9 mm
Filterwheel + filter (+1mm) = 23mm
Atik 4000 : nose is 32mm + 16.5 mm = 48.5 mm

total back-focus distance = 160.4 mm

This is 14.4 mm longer than recommended by Celestron (ca. 146mm), but stars are round to the edge of the chip. So, it is obvious there is latitude for not having the exact back-focus published by Celestron. However, if I add an Atik OAG I am afraid I will be out of luck (another 24 mm back-focus).

So, if I have calculated correctly (perhaps not), I am looking at the "nose" of the 4000 (that 32 mm piece on the mechanical drawing). What is its function? And could I recover 32 mm with a custom part and discard the nose completely?

I have to admit, the camera is 2 hours away in the observatory and I have not carefully inspected the nose part.

Thanks, Ed