How much of a difference would I see in a compression test at 5280ft?

I've been playing around with a used B18 that looks like it just had a rebuild. I can see the hone marks still inside the cylinder (through the spark plug holes on a sunny day) and new valve seals. I'm getting 125psi (warm engine) at 5280ft. The spec in the greenbooks say 170-200psi. I know the difference in atmospheric pressure at mile high is about 20 percent less than at sea level. Can I assume that the compression numbers would be off by 20 percent, and adjust my readings by that amount? So, would that give me a 150psi reading at sea level for my 125psi at mile high? Or is there something else that needs to be calculated in to it?

Paul