Incidentally, what is the lowest that commercial traffic is likely to be above the base of CAS?
Is it 500ft, or is it 1000ft?
Surely as 1000ft is the min separation, and silent non-XP GA traffic could be 1ft below the base, the commercial traffic should be 1000ft above and not 500ft as someone suggested.
This will really get some people going but guess what I have (purely incidentally of course) found to be the most accurate indicator of the local QNH? The GPS altitude. Never more than 100ft away from the altimeter when the latter is set to a known airfield elevation (on the ground) and usually within 20ft. But to be fair I have a £5000 GPS with a rooftop aerial.