"...spraying or misting a tarantula's cage to increase humidity is mostly an exercise in futility and a waste of time and effort." -- Schultz and Schultz The Tarantula Keepers Guide
If you do a little studying on air flow and weather patterns you will quickly find this to be true.
If you have an open enclosure, allowing for good air flow between the enclosure and the outside air, the mist you spray will quickly evaporate, being assimilated into the outside air, and the air in the enclosure will match that of the air outside. Not to mention, it will annoy and stress your tarantula. Unlike your reptiles, tarantulas prefer not to be wet.
If you have a closed enclosure restricting air flow, the water dish will evaporate and be trapped in the enclosure which in turn will raise the humidity.
Keep in mind that reduced air flow also encourages mold growth, while increased air flow reduces humidity; it is a balancing act to say the least.
I recommend you find that sweet spot that will allow for good air flow but, restrict it enough to hold the humidity.
Just my 12 cents.