It is trial and error to a certain extent, but at the same time, you don't want to try to reinvent the wheel either...a lot of people have had bad luck with keeping ball pythons in large enclosures and it doesn't make any sense to dismiss the experience of MANY other keepers on that basis...

A ball python is going to pick one or two areas of the large cage and stay there, leaving the rest of the huge enclosure unused. Moreoever, the added space is going to make the animal feel more vulnerable and less likely to see out optimal temperatures. If your snake happens to pick an area it likes on the cool side, it will choose security over warmth and never be exposed to the higher temperatures of the warmer side.

Just my .02...though it sounds like you've already got it all figured out....