Here's an interesting calculator to try:

http://www.rbracing-rsr.com/comprAdvBMW.htm

Beside change in altitude, it takes into account valve timing etc. What I can't understand is why the entry for rod length - stroke is stroke, right? One thing I do know is that a piston spends a bit more time in the lower half of the stroke than the top half. The shorter the rod, the more that difference. But that shouldn't affect simple compression ratio.