If you're using one of those analog hygrometers, it should be calibrated using a
kit. Once you establish that the hygrometer is working fine, I would suggest attaching one or two 2.5" fans right outside the vent material. It should bring the humidity down to about 60-70%, provided you only mist occasionally.
The vertical conversion you have is much better suited for dart frogs or some other animal that requires less ventilation and more humidity. Dane from Junglebox sells one that has a bit larger vent area (~3.5"). Gray tree frogs are tolerant of a wide range of conditions, but tend to prefer a humidity of around 50-60%. Tree frogs in general require good airflow, else they can stress out or develop bacterial infections.