There's a sticky somewhere on this site that shows how to cover your screen top to minimize moisture loss. Basically you use three layers of aluminum foil, shiny side down to the screen, taped on with duct tape leaving a hole on the warm side about 4-6" square for circulation. Because I use a night bulb lamp, I added a section of thick clear plastic in the middle to let the light in but keep the moisture from getting out.

Normal everyday humidity should be 50-60% measured on the cool side with a digital hygrometer. When snake is in shed, that should go up to 60-70%. For those times, either add a humid hide if you have the room or lay a moist paper towel over most of the opening left on the screen and mist it along with the tank to keep humidity up.