Humidity isn't all that big of a deal, the only thing the snakes REALLY need it for is the shedding process, which you can see coming, and just bump up the humidity a bunch when they go into shed.
It's been seen quite often where balls will get respiratory infections when humidity is constantly too low.
OP: at 40% like the you said, with the extra dish and the misting I think everything will be just fine
1.0 Normal - Maynard
1.0 POG - Victor
0.1 YB - Diana