When you are hardening off grafts and dropping humidity, your goal should be to drop the humidity 15-20% per day.
I used an ultrasonic humidifier I got from Walgreens inside a 5'x5'x7' tall grow chamber for 4 years. The humidifier was on a timer and the outlet for the fog perfectly fit a 1.5" or 2" PVC fitting which pipes the fog into the chamber. The tank is 1.5-2 gallons and that tank didn't need to be filled for 10 day or more depending on the stage of graft healing.
I have since bought a Green Air RHC-R Relative Humidity Controller and love it. It's $300 retail, but I have found it highly accurate and it's very easy to lose $300 in grafts at $10-$12 retail. I also tried using those $10-$20 little grey and orange humidistats you can find on Amazon and had some disastrous results with the sensors frying out on several. If you have a large indoor growing area like a greenhouse or high tunnel or are grafting using a larger healing chamber, you should invest in something that can double check your humidity like a sling psychrometer. It will help you forecast cultural-related conditions and allow you to learn the conditions that help/hurt your plants.
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