Manifold gages can be checked against the altimeter set to zero but as I said before, the instruments are not temperature compensated or easily adjustable.
In fact, they are a pretty poor way of determining power margins where the difference between hover performance and takeoff capability in helicopters is relatively small.
The only answer is a mast torque indicator but I do not know of a piston helicopter that is fitted with one and I am at a loss to know why not. Probably for the same reason that carburettored fixed wings are generally not fitted with carb temperature gages - it would indicate a problem that the manufacturers would rather not have to deal with.