Since you're likely to get different "pressure" readings on the same cylinder from different gauges due to calibration variations, compression pressure is a variable thing. Anything over 100 is good on a stovebolt engine, and the thing I look for is consistency. If cylinder readings are within 10% it's OK, but 5% or less variations is excellent.

I test a compression gauge for accuracy by hooking it up to my air compressor and comparing readings to the line pressure gauge on the regulator. You can even dial up the pressure with the regulator to different values to check the compression tester over its entire range that way.
Jerry

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