If you can switch to a under tank heater(UTH) instead of a lamp(use a thermostat to control the temp), you'd be able to cover most of the screen and keep in humidity. Lamps suck the moisture right out.

Each house would have a different ambient humidity and temp, so you'd have to experiment to see what works for your particulars. Just make certain you don't have anything in contact with your heat lamp to prevent the hazard of fire. If you have pets, make sure a cat or dog can't jump up and bump anything onto the heat lamp also.