I'm assuming your using a tank with a screen top?
When I used mine, I had about 2/3 of the top covered with a thick dark towel and the heat lamp at the gap.
I also had the water bowl half on and half off the UTH so it evaporated and added to the humidity.
Whenever he was in shed, I added a lid with a hole to the top fo the water bowl and that boosted the humidity in there to help his shed.