I believe a reasonably accurate formula for the height difference between two millibar levels is:
h2 - h1 = 221.1T (logP1 -logP2)
... where
h1/h2 are the two heights,
P1/P2 are the two pressure (mb) levels and
T° is mean temperature in degrees Kelvin.
By appropriate substitution (i.e. P1=1013.25, P2=1012.25 and T=15°C=288°K), this indicates that 1Mb corresponds to 27.3ft/mb at MSL in ISA.
I would have thought this would form the basis of calibration for a standard altimeter.
JD
