From the FARS (part 27)
Regarding Performance Requirements:
(d) For reciprocating engine-powered rotorcraft, the performance, as affected by engine power, must be based on a relative humidity of 80 percent in a standard atmosphere.
(e) For turbine engine-powered rotorcraft, the performance, as affected by engine power, must be based on a relative humidity of--
(1) 80 percent, at and below standard temperature; and
(2) 34 percent, at and above standard temperature plus 50 degrees F. Between these two temperatures, the relative humidity must vary linearly.
So, they don't take it into account for anything above ISA+50°F. Pity, because it does have an affect.
And remember that power available for a turbine depends on pressure altitude and temperature and power required depends on density altitude. And they are different!