Humidity in a glass enclosure with a screen top can be a little challenging but there are some tricks to getting it just right. My little trick is to take a towel and get it damp and fold it over along the top of the screen. The towel should obviously be positioned to leave a gap between it and the heat lamp. This should hold the humidity steady and if it drops for whatever reason just put more water on the towel or mist the enclosure. Also if you can try to put the water bowl under the heat lamp that will help the humidity too.

Good luck