If I were you, I'd change my setup a bit. Lowe's sells a couple of things that you'll find useful; a white extension cord with a dimmer switch and an indoor/outdoor wired thermometer/hygrometer. These things will make it much easier to control the ground and air temperature while keeping an eye on the humidity.
Take your snake out of the enclosure and put him in a secure place. A tied off bag inside a tied off pillowcase should be perfect.
First, ditch the cardboard. Clear the substrate from the UTH and stick the wired sensor to the glass right above the middle of the UTH. Stick the thermometer itself to the inside of the glass on the opposite end of your enclosure so onboard sensor is a few inches above the substrate. I tape the external wire to the corner of the enclosure all the way up to the thermometer unit to keep the snake from getting under the wire and pulling it up.
Next, get your dimmer cord out and plug your UTH into it. Set the slide at about midpoint or lower and plug it in. Let it sit for about 20 minutes and see where you're at. Go ahead and let the UTH get to 82 degrees if you use cypress for substrate, a degree or two lower if you use newspaper. As long as you're not using a screen lid, the other side of the tank should stay a few degrees cooler and allow your snake to self-regulate.
A really good tool to invest in if you dont have one is a temperature gun. The thermometers are good for measuring a set location, but a gun will give you a much better idea of what temperature the different areas of the tank are. I like cypress for my little guys so the top of the substrate is actually a few degrees cooler than the sensor suggests it would be. They do burrow so I dont want the temp above 82, but I also want to know that I dont have so much substrate in there that the heat isnt being distributed properly. A temp gun is the only way to check that.
