SR71 & MkVIII,
Two very closely matching responses in such a short time, thank you sincerely to both of you.
I'd always kept the figure "in my back pocket" as it may have had aircraft icing considerations, after considering the Pressure Height, maybe not a consideration for current transport aircraft. I'll leave that zone for the high flyers like SR71.
MkVIII, your tone in this and other posts sounds familiar, so too is that diagram. You weren't the Uni student who nicked my notes were you?
Thank you,
Old Smokey (aka Old Grumpey)