The various legislations from the regulatory states DO consider one standard Relative Humidity when applying the certification rules to aircraft performance.
RH, of course, varies enormously, but be assured that it is considered (albeit a standard one) within the various legislations.
I don't happen to have my various reference materials with me at the coffee shop from which I am replying, I'm sure that by the time I return home someone will have provided the figure.
Regards,
Old Smokey