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A compression check is always done with the engine NOT running. Pull all the plugs, open wide the throttle and take a compression reading. The tricky part is knowing what to look for and how to interpret the results on the test.
Note the number of cranks or "time" it took to reach full compression per cyl. The first "hit" should make the pressure gauge really move up the scale. Then additional compression strokes will build the final pressure up to max fairly quickly. All cylinders should be within 15% of each other, being a new engine I'd be tempted to say 10%. The total amount of pressure per cyl is still going to be hard to interpret, so many variables regarding cam shaft, etc. It's not really a reflection of "compression ratio" per se.
I recently tested my 427. 230-240 psi per cyl. 12.5 to 1 Compression.
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