I don't know anything about this personally, but I know that a few people on here do, and I'm looking forward to the discussion! A simpler method might be to just have your misting system on a timer, and just figure out how often the timer should come on through trail and error. That's what I'm doing with my misting system. My terrarium is totally closed, so the humidity is pretty much always at 80-90%. I mist once every three days currently, but my husband will be installing a very small fan to circulate the air this weekend (hopefully), so then I may have to figure things out again. A totally automated system would be really cool though. Possibly expensive, but cool. =)

The simplest humidity controller I've seen is a piece of sponge (use a real sponge, not chemically treated) on a wire on a microswitch. The microswitch turns on a 24V water valve/solenoid when the sponge dries out. Make sure you make everything very robust. Put the microswitch terminals in some heatshrink or similar.