I hate to bring this back up again after so much work has been done to improve it, but I still see an important issue with it. There appears to be something wrong with the relationship between QNH and the ambient pressure at that location. As an example, I located the airplane at a high elevation airport (SLLP) in La Paz, Bolivia. Using real weather the METAR indicated a QNH of 1037. The altitude simulation variables are shown in the following screenshot:
As you can see, it gives an ambient pressure at the airport of 635.45 hPa for the QNH of 1037 and airport elevation of 13,313 ft. However, for that elevation and QNH, the ambient pressure should be 627.57 hPa. The MSFS pressure leads to a pressure altitude of 12,355 ft, but it should really be 12,670 ft. A QNH of 1037 should shift the pressure vs altitude curve down by 642.5 ft.
If the ambient pressure is really 635.45 hPa at the field elevation of 13,313 ft, then the QNH should be 1048.8.
My calculations are based on the following equations from the Boeing Jet Transport Performance Methods document (with 1013.25 hPa substituted for 29.92 in Hg as the sea level pressure).
This is a stumbling block for us in trying to implement the effect of barometric pressure compensation since what we would be basing it on (the equations above) are different than what MSFS appears to be using.