I have 3 the same size enclosures for my biggest balls. my humidity floats around 50-60% with just my water dish and occasional misting, but that's with my climate in Michigan.
I tin foil the tops to help hold in humidity during shed cycles though, to keep the humidity above 70% At a local pet shop recently, I saw a 200-300g ball with bad stuck shed. They had a digital hygrometer in the tank and it read 45%. If they knew more about the pets the sell, it most likely would have had a good shed.