dublindispatch:
Is 20000 some magic height that the industry has chosen or is it based on average ash height etc?
'The industry' has little or no input to this process - more's the pity.
Ifit can be convinced that this process is scientifically valid (which at the moment is not the case), 'the industry' would rather have charts with more and probably different altitude segments, to help them make their own operational (not commercial) judgement about whether to fly under, over or around.