3" for the smaller tubs, 4" for the bigger tubs but remember our ball pythons have their own room so it's more or less a very stable, temp controlled environment not subject to the swings the rest of our house is. The 4" works fine for our really big tubs but they aren't in a cool room with windows open or anything like that so the cool end is fine. I'm not sure how flex works in a more "normal" household room when it comes to really big tubs like I have our biggest female BP's in. I'd have to go look up what Rich uses in our STC-410 narrow rack but it's back heat not belly heat.