Compression ratio has nothing to do with atmospheric pressure. The actual compression does but not the ratio. The compression ratio is just the volume of the cylinder at the bottom of the stroke compared to the volume at top dead center. I'm trying to do the math right now, hahahaha. And yes before you ask, I am at work
***edit***: the ratio is top dead center until the exhaust port/valve starts to open. Some other things may apply, but in general this is it.