Yes, good advice, check your gauge hose and seal, also the check valve in the gauge could be leaking bleeding the pressure off. 2stroke outboards generally have less compression.
Your doing the math basically right, but the concept and method of the test is not the same. Think of it as "dead heading" an air compressor for a peak pressure reading at equilibrium, that will determine efficiency. To make your idea into a valid test would require just one rapid cycle of the piston. Also, you would have to take the volume of the hose and gauge into consideration, both difficult to acheive and why this method not practical.
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