How much Mixbus headroom above zero?

[mixsit]

There is apparently some extra headroom in the ACKUS mix system, but I would like to understand better how it works.
What I think this means is...
With the master fader at zero, and the master meter in 'pre-fader' mode you can have your mix can go into the red, but if you pull the master fader down so it is out of the red with the meter in 'post' mode, you can have still have an un-clipped output.
Is this ok so far? If it is, then my question is -how much headroom is alowed?
I gererally tend to build mixes well below zero, but there are times where the mix is complete and the choice is to trim all the tracks down or start adding limiter plugs -added processing the mix does not need.

Thanks in advance.
Wayne

[moskus]

For some reason, 0dB in Sonar isn't clipping (actually 0 dB). Sonar will only clip if the red mark over the meter lights up. I don't know how much the headroom is but that should be easy to test...