Baader coma corrector distance

[wimvb]

I'm about to place an order for a Baader Mark III coma corrector. According to the specs, it has/needs a back focus of 55mm. The flange focal distance of my camera is 45.5 mm, which is right in between Canon at 44 mm and Nikon at 46.5 mm. The T-ring will also add some distance.

My questions are:

Will I need some sort of spacer or other additional hardware to get the distance right?

How critical is the distance? I vaguely remember reading that the tolerance is +/- 1 mm.

[Costas Soler]

What model telescope are you using with it? I have an Orion Newtonian astrograph and use the Baader mpcc iii and I don't need a spacer. The coma corrector is a very solid piece of hardware, and I regret nothing with it. I use a cardboard bahtinov mask to focus and it's been pretty much perfect so far, no spacer needed. 

I took this wide shot of the lagoon nebula using that corrector. As you can see, there is basically no coma

[wimvb]

Thanks for the info. The scope is a Skywatcher 150P.

Great image, btw