The main thing you are looking for is consistency between cylinders. Accuracy of the gauge is handy but if you have a significant difference between cylinders you have a problem.

Compression testing on a diesel is done DRY with fuel lines disconnected. Adding oil to the cylinders to get a wet reading is useless and will give you false readings.

A quick, rough check of your gauge's accuracy can be done using shop air.

ALL gauges, including Snap-On, OTC or any brand, will suffer from some inconsistencies. It is just the nature of the beast. Barometric pressure, humidity, etc. create variances.

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