I think the book says"within 15%" I'd say within 20 lbs. If the rings are bad, you can test for that by putting a squirt of engine oil in: If the compression comes up significantly like 20 lbs it's rings. Valves are usually leaking, but it's easy to do the head: Just take it off and bring it to a machine shop. Watch it: They like to surface the head. Don't have them do it if the head is not warped much. Measure the thickness several places: It should be parallel. If they machine it too thin, it's scrap. It's aluminum and will bend easily .010/.015 inch when you torque it down. They are all curved when they come off. The gasket is quite thick and will take up some uneveness. Check your valve clearance before you do anything.