You'll forgive me, but in post #175, it wasn't clear that you were talking about 40,000 feet at all.
I'm not sure why you keep repeating this, since nobody seems to disagree with it. If you mean to say that "same at 10,000" automatically proves "same at 40,000", then we disagree.
I'm afraid I don't understand this. I can gain altitude, even without accelerating. (?)
When I followed your suggestion to calculate the TAS of Mach 0.9 at different altitudes, I was suddenly reminded that above 30,000 feet, there is an inversion - the TAS for a given Mach begins to decrease with altitude, and then to stabilize at around 35,000 feet. I began to wonder if maybe Lock On is not modelling the aircraft incorrectly, but rather - the air above 30,000 feet?
I didn't test the idea further.
If you mean my "Average error" - that is not a simple function, because the angle of the climb changes from the start to the finish of the climb. At low altitude, distance vs. time is small, while at high altitude, it's much larger. But the "averaging" only looks at the total time and the total distance. They are not proportional, so they have different percentage errors.
Anyway, the "average error" is not a very useful calculation. If you look at the green bars in the charts, the specific errors near 35,000 and 40,000 feet seem very similar for both time and distance, as one would expect.
-SK