Altimeter setting procedures
Do you change altimeter calibration to STD exactly as you climb through transition altitude and to QNH exactly as you descend through transition level? Or do you change to STD as soon as you are cleared to climb to a Flight Level and to QNH as soon as cleared to descend to an altitude?
I have always done the latter but at a recent simcheck I was picked up on it and told to do the former. Anyway, does it actually matter from the point of view of separation?