There are some varing opinions on this subject. Most people agree that 60% is right in the "ball" park. My racks stay around 60% but I keep my tanks a bit higher (70%+).

I think the reason most people go with 60% is because that's about the lowest humidity that BP's can handle without too many problems (RI etc). Higher humidity levels can also lead to problems if things are not kept very clean.

However, my reasoning is that BP's in the wild live in the ground. Have you ever slept on the ground? It's very very humid. And one has to question why the conditions we keep our snakes in is not appropriate for incubating the eggs they lay. In the wild they LIVE in the conditions appropriate for incubation (very high humidity).

My experience is that snakes kept in 70% humidity are healthier in general than those kept at 60%.

Just my .02