What size under tank heater is appropriate for a California king snake terrarium?

I just received a young California king snake and have just purchased one of those snake habitat kits by All Things Living. The habitat kit included an under tank heater and I was wondering if it was the appropriate size for the snake's enclosure (I have him in a 29 gallon aquarium which has a 30x12 footprint). I've heard from different sources that the under tank heater should cover 1/3 to 1/2 of the bottom of the aquarium. Is this correct?