You measure the temps from the glass as the snake can move the bedding out of the way and lay right on the glass. That being said, you HAVE to have a rheostat or thermostat to regulate and control the heat manually. It's one of, if not the most, important pieces you have. This is nothing something to be cheap on and think you can get away with not having. I made my own dimmer for all of $7 each which is easy to do. If you are able to follow simple instructions you may want to check out doing them yourself.
http://www.cornsnakes.com/forums/showthread.php?t=93490
That's an example of what I'm talking about.
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