Buying New Tank

So I am getting a new tank for my BP and I was curious if he would be good to go in a 30 Gallon Long? I know that is the biggest they should have, but I was just wondering so I do not have to buy another tank in a couple months if he outgrows the 20 I would buy.

He is ~31 inches long and ~580 g

Also, what size UTH should I be getting?

I have heard 1/3 of the tank, 1/2 of the tank. What is the best coverage?

really they could live in their 20 and be fine, they actually don't mind reasonably cramped spaces, but you want a hot spot (88-90F) on one side or the other, as far as coverage, 6-8" should be fine.

My BP is about the same size as yours, and is currently in a 20L. I just posted a couple days ago about my tank getting cramped after hides, water dish, decoration etc. IMO a 30 would probably be a better fit. Just be sure that there isn't a lot of open space, so that your BP feels secure moving around. Even just a $2 leafy vine from Wal-Mart's craft section would look good and provide good cover for your snake.