It is altitude dependent. Not the actual C/R but what a standard compression tester will give you in PSI. So a healthy motor at 300' might read 150 psi then take that measurement again at 10,000' and you might see 108psi. Its still a healthy motor at 10,000', its just the air density is significantly lower at that altitude.
Also, a ported motor may read lower with a compression gauge than a non-ported motor because the cylinder isn't "sealed" as long during the compression stroke, but it will flow more air and make more power.
I had an Indy 500 that read 80 psi cold at 7700', and it was healthy (just broken in from a rebuild actually). Never did check what it read at low altitude though.
Just some food for thought.