The calculations below show actual feet/mb for various altitudes assuming ISA:
Zero ft 30 ft per mb
5000 ft 30 ft per mb
10000 ft 31.45 ft per mb
20000 ft 36.25 ft per mb
30000 ft 40.84 ft per mb
40000 ft 47.84 ft per mb
50000 ft 52.75 ft per mb
60000 ft 62.96 ft per mb
What is your source for these numbers, or the basis of your calculations, please, WBS?