Why mist at all? If you need to increase the humidity just pour water onto the substrate thus soaking it. I have said it 100s of times....misting evaporates too fast and will not do any good therefore is a waste of time but I dont think anyone pays attention when I or anyone else says that.
If its not a sling, misting is not effective enough to raise humidity overall. You can just pick a side of the tank to pour water onto and let that side stay wet and the other stay dry. I personally do corners of tanks. This soaking takes a lot more time to evaporate and works much better then misting. (and this applys to all species requiring higher humidity.)
I inject water into the substrate with a very long small diameter tube via a 3cc cattle syringe, at various depths, where needed. Works great! I have never misted, I gathered misting doesn't really do any good, from reading the posts on here.