I don't really like the purple/blue bulbs for reptiles. The red ones work better, and at night the snake can't see the red, so it doesn't disturb them thinking that there's still light outside. The red ones also seem to put out more heat for the buck. For a typical 10-20 gallon aquarium, I don't generally go over a 40watt.
One thing to mention, if you can feel the heat on top of the substrate from the light or UTH, its too warm. Optimal temp is 82-84, which is well below your body temp. You can only feel temps above your body temp, really.
You need to turn the light and UTH out, get a thermometer and lay it on the bare glass of the tank for 30 mins and record the temp. Do the same for on top of the substrate, record that temp. Then turn on the light and re-measure both. Same for the UTH. Then measure with both the light and UTH on. Anything above 85 is really too warm.
Some of the newer UTH's don't seem to get as hot as the older models. But most of my corns burrow, especially the young ones, and the glass under the substrate can get quite hot.
Another way that works really good for keeping the bottom from getting hot if you don't have a thermostat or rheostat, is to stick the UTH to a ceramic tile, elevate the tank, and just set it on the table. That way it warms up the bottom, but not to the high temps that you get when its directly mounted on the tank.