compreshion ratio calculations

I got to thinking about calculating compreshion ratio from compreshion tests and this is what i came up with. I think this is corect based on no leaking parts. atmosphereic preshure at see level is 14.7 so a 10.5 cr moter should have 10.5x14.7=154.35 so the gauge should read 154.35 -14.7=139.65.the minus 14.7 is 0 on the gauge is really 14.7 at sea level.or you could figure it this way. your test shows 160psi. if the gauge reads 0 to start then 0 is 14.7 axualy so add 160 + 14.7=174.7divided by14.7=11.8 for your compreshion ratio. well isnt that fun am i right i think so .i know this is based on perfect cercumstanses. second opinions anyone