Octane and Compression
I don't think that sounds right.
Regardless of altitude...
If the car does not ping on 85 octane ( just a number) but you put 90 octane (just a number) in it, the result will be that it will not ping.
Now on the other hand if the car doesn't ping on 90 octane (just a number) but you put 85 octane (just a number) in it the result will be that it might ping etc.
Now as for altitude, there is less oxygen in the air and a lower octane will not ping just as good as a higher octane at sea level.
I would just use the highest octane I can buy that does not ping. Heck if you could get 104 octane it would never ping regardless of altitude or load or temprature