Do I need one of these?

[Vox45]

You want this to go between the Canon and the back of the reducer:

http://www.firstlightoptics.com/adaptors/skywatcher-dslr-m48-ring-adapter.html

It will give you the exact spacing - it can be split in to two pieces by loosening three small screws so that you can rotate the camera relative to the scope once you have everything screwed together.  It isn't ideal though since you usually want to rotate the camera for framing once you have it on the target, and you definitely don't want to be unscrewing those tiny screws in the dark - easy to drop, hard to locate and tighten up in the dark, easy  to damage the heads, might drop the entire camera on the floor, etc.

The FLO adaptor comes in at the other end of the reducer (http://www.firstlightoptics.com/adaptors/flo-adapter-for-skywatcher-focal-reducers.html) - you put that on the front of the reducer and you can then use it in the 2" focus tube as if it was an eyepiece - so you can rotate the camera more easily.  I wouldn't though personally, the 80ED has two 'comedy thumbscrews' and I would not trust my DSLR and reducer to them, still less anything heavier (even if you replace them with bigger thumbscrews there is not much meat on the threaded holes and they tend to work loose by themselves). Plus the two screw design can make the camera sensor not be orthogonal with the focal plane.

The big plus of the adaptor for me is that it has a 2 inch filter thread on the front of the nosepiece for my LP filter.  The only other options for putting an LP filter in the train are to screw a 2" filter between the back of the reducer and the Canon adaptor (which affects the spacing and creates elongated stars in the corners of the frame), or to go for one of the in-camera clip filter types (which limits the choice of filter and means it can only be used with a Canon).

What I did in the end was go for a Baader Click-Lock like this: http://www.firstlightoptics.com/adaptors/baader-click-lock-2956256-m56-celestron-skywatcher.html

That screws on to the silver thread you have now found, and then you slot the FLO adaptor in to it and rotate the locking handle - rock solid connection and nicely orthogonal due to the brass compression ring mechanism.  Makes it a breeze to rotate the camera.

Bumping this thread

Your answer save me a lot of money ! I do have a click-lock already so I would only need the flo adapter.... but can this adapter be used as a nosepiece WITHOUT the ff/fr ? I mean, can I use the adapter and screw it directly into the T-Ring thus giving me the possibility to use it as a regular 2" nosepiece when I don't need the ff/fr (lunar/solar for exemple)

I was also looking at the Baader protective ring (a very elgant solution) but I was worried that the IR filter between the DSLR and the FF/FR would change the spacing so I'd rather go with the IR filter in front of the reducer.