Terrarium Heater

I just got a young king snake and California have just purchased one of these kits snake habitat for all living beings. The kit includes a habitat under tank heater and I was wondering if it was the right size for the snake enclosure (I have 29 gallons in a tank which has a size 30x12). I heard from different sources that the under tank heater should cover 1 / 3-1 / 2 of the bottom of the aquarium. Is this correct?

if it came with the kit that I u, then that should be fine, just make sure that its under the skin of a snake box on the warm side ... hope this helped ... good luck with the new snake =)