Jim, 20 quick hand flips are not enough. Your only spinning the engine over 1 to 2 times at best. All readings should be made at cranking speeds with the throttle open. A warm engine will give a better reading also because the clearances are closer to what a running engine would be.
Also, what about ringed engines? (your's isn't) Volfy is right about dykes rings, so it would have to be checked while spinning over at close to running speeds just to get a reading. What I'm saying is: If you check every engine the same way, you will get a more consistent readings from engine to engine.
Suptter