After some checking around I found:
assuming standard atmospheric pressure is 14.7 PSI the CR can be calculated from that info
for example if measured 135 PSI the CR is 135/14.7=9.18
This is only truly accurate for sea level but for what I wanted - expected ball park number to see if engine in good shape works for me. If I checked an engine that was supposed to have 10:1 compression I would expect ball park of 148 PSI if in top shape.

Thanks for the response techinspector.