That is not an easy question to answer, there are many variables. things like air temperature, atmospheric pressure and humidity vary all the time and affect the numbers you would get from a compression test. Then there are mechanical considerations, like throttle position (should be wide open but is it really wide open?) wear and engine temperature. Say you have a Cooper S with a 10.5:1 compression ratio. In a perfect world, you could multiply average absolute atmospheric pressure, 14.696 psi, by 10.5 and get 154.3 psi, but you are measuring your compression with a gage, gages read one atmosphere less than absolute pressure, so subtract 14.696 that equals 139.6 psi, but no cylinder seals perfectly so the pressure is going to be something less than 139.6 psi, depending on your engine. You are likely to see a figure in the 120 psi range. Generally, if the difference in pressure between the cylinders is less than 10%, things are good. Dave