Covering as much of the screen will help out a bit, and so will moving the water bowl more towards the heat. What are your actual temps? And are you using any kind of thermostat for the UTH? Between the heat lamp that you have and a possible unregulated UTH you may have your temps a bit too high and it is drying out the tank to quickly. Check all your temps and adjust them if needed and then check out where your humidity is.