If you're going to house FBTs then you should have a water section in the viv (making it a paludarium according to the definitions
) because FBTs do much better with a water section to swim in. Since they can swim in the water section, I'd say you couldn't get them much more humid than they'd be soaking wet
so there'd be no need to worry about your humidity. If you wanted to keep FBTs but didn't want to make a water section then I'd suggest at least in the 80 - 85% range.