I have an example for true altitude calculation. The given Information is QNH: 983hPa, cruise in FL 85, ISA -10°C in this Alt.
The math and calculations are not hard and i also get to the correct result, but my Question lies in understanding the principles.
So to get the height difference from my QNH (MSL) and 1013hPa i simply take the difference of 30hPa x 30ft. and get 900ft. of seperation.
Now to the Question: Why is it correct to calculate the difference from the QNH, so actually a "virtual pressure", to a true temperature corrected pressure level of 1013hPa? Wouldn't i need to calculate it from the actual temperature corrected pressure at MSL?