Get a temp gun. It's $30 you wont regret spending.

I setup my tanks by "feel" with under tank heaters and a ceramic heater in a dome. I though I was pretty close but didn't "know". So, I bought a bunch of the sticker temp guages because they were cheap. The sticker temp guages are completely useless. My snakes are in 55G aquariums with the sticker temp guages low on the back wall at both ends of the tanks. They don't register at all on either end -- temp below 69F according to them.

I keep my house on the cool side -- thermostat set to 68 in the winter. I was freaked out when I put the stickers in. I thought I was freezing my snakes. I got a temp gun. According the temp gun I'm at 88F on the hot end and 72F on the cool end. I'm sure if I had tried to adjust the heat using the stickers I'd have cooked the snakes.

I got my temp gun at Home Depot and definitely appreciate it. It takes just seconds to verify temps and it's an instant reading.