Shawn.
Some of the frustrations of power determination on piston engined helicopters:
Manifold guages whose reading does not correlate with the altimeter pressure reading.
Manifold guages that are not temperature compensated. Climb into cockpit when console has been cooking in the sun for hours - MAP over reading by more than one inch.
Same day with high humidity which is not scheduled in the flight manual.
Bugs on the blade leading edges in spite of cleaning just before wind up.
Weight schedule incorrectly recalculated from the "as weighed" figure and a lot of c..p under the seats etc, etc.
Some helicopters which in spite of engine/blade changes are always the dogs of the fleet.
Result: The only test is does that aircraft hover with the appropriate MAP margin on that day at that time and this may be widely different from another aircraft which is apparently in all respects similar.
This is all before the variations due to engine/blade deterioration is considered.