dry tests may be irrelevant as creepage always degrades with rising %RH , hence why bushings often have so much invisible but audible corona in morning dew situations and beyond.
I’m afraid that you don’t find such a relation, valid for all technical cases. The bushings are and will be always a sensitive element in a high voltage equipment. Even if the bushing are built using grading capacitors (capacitor bushing), the zone with maximum non-uniformity of electric field is around the mounting flange. The relative humidity does not have, of course, effect on internal PDs. But the surface condition (polluted, wetted), mainly around mounting flange strengthen the electric field and corona can occurs. But even in that case the breakdown is still far. Only if sliding discharges appear (some relation exist, but not for hollow or capacitor bushing and for dry condition) or dry band arcing, the breakdown coming soon. And generally, the corona discharge which could appear also on the water droplets or dew is first heard and then observed with naked eye. So, for the external surface of bushing it’s more important the dew point vs. relative humidity.
The capacitive impedance is highly affected by the polar content of moisture or humidity. This affects both low voltage transistor SMT chips with dust from high-speed fan-cooled PC's that accumulate dust as well as surface contaminated bushings in UHV towers serviced by maintainance using faraday-suited workers in helicopters with poles to clean the bushings on >500kV lines. The lower voltage lines are just tolerated with corona self-cleaning.
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