I'm no expert.............
Boyle's law P1V1=P2V2 means that increased volume = decreased pressure.
But logically, 24psi is 24psi whether set at sea level or at altitude. But the thing is - 24psi set at sea level will be less at altitude according to Boyle's law. And vice versa.
So the pressure will decrease as the volume inside the tyre increases as atmospheric pressure decreases with altitude.
So in the same way that a weather balloon expands with altitude until it pops, I would expect a tyre to increase in volume with altitude.
Set your spare pressure at sea level, try to keep the spare at the same temp and recheck the pressure at altitude. Or vice versa. I don't think it will make much difference.
Warning! - this might be rubbish.
