@411A,
no disrespect, the AFM is obviously the first resource when checking for optimum altitude. And I do check numbers in the AFM once a while
However, me thinks elpilote is asking where these numbers/graphs are coming from. What´s behind them. I am sure the CAA/FAA won´t be impressed when answering that sort of question on a test with: 'Well, check the AFM'.
@mutt,
what I was trying to get at was that at one point, be it Mcrit, Mmo or some other speed in that ballpark, the increase in drag (specifically wave drag) will be prohibitive for the TAS required to maintain optimum AoA.