one way to make a tank have good humidity is to place something over the screen a plastic shopping bag or trash bag make sure you have a good area open for air flow though. get a thermometer gun (some pet shops have them also lowes and home depot have them) yes they cost more then your in tank thermometer but they give you specific temperatures of the surfaces not just the ambient air temp in the area of it. and the under tank heater will help more then the lamp will. your lamp is heating the top of the substrate, that heat then bleeds off into the air which will become lighter and that hot air will then leave your tank. the under tank heater will heat up the tank and then penetrate to the substrate and warm it from the bottom up giving it a better over all temp. when you get the temp gun make sure that you push some substrate back away from your hot spot and get both the bottom temp and the surface temp.

you have 3 ranges of temp to monitor the cold side surface, which should be low 80's the hot side (both under and on top) which should be 88-90, and the ambient which you want to have in between. the only reason to keep the lamp would be if your room is cold and you need to heat the ambient while having the hot side a little lower then you would keep it at with out the lamp.

please any one with more information point out anything i am saying that is wrong as i don't want to give bad information but this is what has been explained to me.