I am not satisfied with the values of altitude above sea level that I can see on the Crazyflie python client. Sensor seems to be really unprecise and a look at the firmware shows a very costly formula, using pow function. Furthermore, I am not sure that the formula is correct and I replace "standard atmosphere" with a linear approximation. Here you can see the formulas in a human readable mathematical language:
The difference is only a few meters. This is not a problem to althold assistance which uses differences.
Regards.
Jean-Claude