Get a thermostat, you wont be sorry, dimmers suck.
The room temp rises and so does the tank/enclosures temperature.
The room temperature drops and so does the snakes home.
Ten bucks more and you can have a thermostat and way less worries.

Flexwatt is very easy to wire up.
A $2 extension cord, cut off the non plug in end and put the clips on the wire and clamp the clips onto the flexwatt and plug it into the thermostat.

Make sure you have a probed thermometer for reading the flexwatts temperature directly on the hot spot inside of the tank.