From all the info I have been able to gather you should remove the valve at the threaded end of the hose that screws in to the plug holes. I got higher readings when this was removed. While cranking be sure to hold the throttle wide open. The gauge should have a release valve, you want to hold that open as well since you aren't looking for a peak reading. Be sure to warm up the engine first for a more accurate reading.
The rotary specific testers are great but extremely pricey. I used a regular compression tester and feel it gave me a good indication of the engines condition. Best of luck!
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