Crab: you've 'got it'!
Normally aspirated piston engine is 'charged' by atmospheric pressure.
Therefore at Full Throttle (FT) it would be preferable to achieve a DA by a combination of HIGH pressure (LOW PA) and High temperature rather than Low pressure and Low Temperature.
Since the atmosphere, as far as most helicopter pilots is concerned, is 30000ft high and 30"Hg pressure it must reduce at 1"Hg per 1kft.