I'd suggest getting a digital thermometer/hygrometer that's on the ground where your BP is. The problem with the circle analog gauges is that they're not accurate and it's not doing you any good all the way up there. You can pick up something like this at any hardware store.
Also, here's a good sticky regarding 20 gallon tanks:
http://ball-pythons.net/forums/showthread.php?t=201740
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