To find the value of 1mb pressure change at a specific alt in feet
96 x T
-------------
Pressure
Where T is temperature in Kelvin, Pressure is in millibars.
so at MSL (ISA):
96x288K
---------- = 27ft/mb
1013mb
At 20,000ft
96x 248K
--------- = 51ft/mb
466mb
At 30,000ft
96x 228K
---------- = 73ft/mb
300mb
Remember pressure does not lapse in a linear fashion with height.
All the best,
Tom.
