Function nws_pressure_altitude_ft 

pub fn nws_pressure_altitude_ft(station_pressure_hpa: f64) -> f64

NWS pressure altitude, feet, from STATION pressure in hPa.

The published NWS station-pressure equation (https://www.weather.gov/media/epz/wxcalc/pressureAltitude.pdf):

PA_ft = 145366.45 * (1 - (P_hPa / 1013.25)^0.190284)

Station pressure, never an altimeter setting (MBA-643): a QNH already corrects to sea level and would report the pressure altitude of the sea rather than of the firing point.