I have the same questions as above but my bp is in a 20gal long and I had the same problem. My screen is covered in duct tape and i have a human heat pad on warm side and a over head 75 watt red bulb on cool side which i use to have both on dimmers. My tank gets heat by the heat vent and the a/c vent and I was always adjusting temps which worried me when I would go away for the day or even at night when asleep.
So I invested in a ESU electronic temp control ( 27.00) ie a T-stat which works pretty good. I have it hooked up to the light and then I went all out and got a Helix control ( 120.00) for the warm side and now my temps stay at between 80-82 cool and 90-93 warm and I dont have to worry and it maintains good heat.
Get a t-stat you dont need to spend the $$ like I did on the Helix ( the Helix is for when my AP cage finally comes in with belly heat routed in ) the ESU works good.