How to get accurate temperature with probe thermometer?

I have a 125w bulb in a 55g bd cage. Before I get him, though, I'm trying to get the correct temps. but I put the probe on the piece of slate I have for basking and it was in there for around 3 hours before I turned it off and it was still gradually raising, slowly. The problem is, how do I know what the temperature is. The temperature constantly raises and never settles. It's about 18 inches away from the basking area. I turned it on a few minutes ago when I got home from school and its up to 97, but I took the probe off the slate, so it would take the temp of the slate. It's still doing it though. How do I know what the temperature is? I'm confused.