Engineers and meteorologists have solved this.
I meant the people in this thread!
There is a variation in density (even for a given MAP), back pressure and throttle losses all going on. The principles are well described.
Where are these principles
'well described'? If you know, please share!
You had the density bit right to begin with, it's just that you hadn't considered the other two things at that stage.
What are the two other things I had to consider, other than the density?
No. What ft showed was that if you take pressure and density from sea level and apply the ISA derived correction for a given altitude (which accounts for gravity, radiation, convection, humidity etc), when you put them back into the gas law, lo and behold you can get the ISA correction for temperature at that altitude!
Do you realize that's what I said? The 'ISA derived correction for a given altitude (which accounts for gravity, radiation, convection, humidity etc)', as you stated, would be the pressure and density at that altitude. If you plug that pressure and density change from SL into the ideal gas law, you'll get the temperature change at that altitude. You just expanded what I had said.