If memory serves there is a handy formula for working out feet per millibar:
Assume Temp. 30C, Pressure 1020
Formula:
96 X temp. in Kelvin i.e 30 + 273 = 303
------------------------
Press. in Mb
Therefore
96 X 303
---------- = 28.51....... Feet/Mb
1020
Looks nice anyway. Should work for all combinations of temp./Press.
"and I think the formula took several pages of A4". Fear not and enjoy.