Instead of just misting in the tank and getting everything wet including the hygrometer which is why it could be showing so high. Just mist the substrate and the heat will make the water evaporate and cause humidity.

Like Brad had mentioned covering the screen lid with contact paper or some duct tape will help keep the humidity in. Since hot air rises some of the water in the air will rise too right through that lid.

I also recommend getting a digital hygrometer like from Walmart or Target as these are more accurate. I like them because they keep track of Humidity and can track temps on both sides of the tank with the use of a probe.

I hope we helped.