Like Suptter say's

Record your readings and keep a log of them for each engine. Always do it the same way like with the starter and with the throttle wide open.

When you start getting lessor readings, then you know something is happening.

When you read in an automobile repair book about compression readings, they don't always mention what pressure you should be getting. They will just say that all the readings should be within 10 lbs of each other.

Enjoy your gauge,

Jim