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20 is probably good for a 3 cylinder. You could try reducing the idle speed some and see if it goes up any.
No way you will see 28 inches at idle, not enough atmosphere to create combustion.
Higher compression, more cylinders, lower idle speed, as well as highest atmospheric pressure will give you highest readings at idle speed.
Best acceleration efficiency will probably be at 1-2 inches.
regards
Mech
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