Flexwatt sizes

Hey all-

For those of you with tubs, for your hatchling, sub adult, and adult tubs, what size flexwatt do you use for each?

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.