What size heat mat should I buy?

[Scubadiver59]

For a 30gal tank...a 20-gal size should be sufficient since you'll want to keep your hide small as possible and yet allow the snake to squeeze into it.

You put your thermostat probe between the pad and the glass to ensure that it doesn't get too hot. If you don't have a laser temperature gun, get one. That way you can lase the glass on the inside of the tank and confirm that your thermostat is working correctly. Since Kingsnakes are burrowers, lase the glass and not the top of the substrate. You can use a temperature gauge or LED thermometer on the cool side to track temps.

If you really want to ensure that all your temperatures are correct, basking temp and UTH temp, you'll really need a two port thermostat (i.e. Spyder Robotics Herpstat or Vivarium Electronics, or similar type) to monitor and control the temperature of your UTH and the basking light. You can always move your basking light (close to far) away from the cage to adjust the temperature, but the thermostat can throttle the electricity and keep things under control. Now then, some lamps might not be able to be dimmed/throttled, so you'll have to check the lamp specs.