The general suggestion seems to be the heat mat should cover between 1/4 and 1/2 of the tank (around 1/3, I suppose?).
A few more things to note:
It's good you're getting everything figured out before you get your snake! Many folks don't do what you're doing.
Tanks are notorious for being hard to maintain proper humidity. You'll need to take special measures to convert your tank into a happy BP home. Fortunately there's a really great tutorial on these forums. I highly recommend, if you're committed on using a tank, you follow it to the best of your ability!
And make sure you get a thermostat to regulate your UTH. If you don't know the difference between a thermostat and a thermometer, or why it's necessary, definitely keep bringing us your questions!
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