I have a room humidifier but only run it in winter and early spring. You can get a plastic covered shelving unit to really focus on humidity for a reasonable price. Check Amazon. They are made for plants etc like a green house effect.
As far as monitoring humidity, I have a half dozen of the humidity guages but they are notoriously inaccurate. Two different ones will show up to 20 % differential. I look at them but don't obsess over them.
Hydration is best managed with food and occasional face (not body) misting. Better to drip through a lid, or mist a side of a acrylic or glass enclosure.
When I first started the hobby years ago I was a heavy mister LOL! But I was having far too many heart breaking mis molts... You would think water is the right thing to help a PM loosen up their skin but what I Shortly realized was it changes the consistency of their skin if done too close to a Molt and they get stuck. A mantis will excrete fluids leading up to a Molt as their natural preparation for molting. Spaying them with water can change the PH of their skin...
I have almost completely eliminated mis molts by doing the things mentioned above and providing course fabric surfaces for them to Molt.
I will say that Cold climate states present a greater challenges for this hobby because winter months require artificial heat and heaters dry the air. So humidity can reach risky low levels.
Humidity also can introduce other even greater risks to PM's by allowing conditions where bacteria, and mold can grow, causing fast death.?
If you mist, provide a fan to circulate air whereby the water evaporates with in an hour.
Cover tops or sides of enclosures with seran wrap, but keep air flow at a good level.
Of course each species has its own humidity requirements as well. Google temps by month and humidity levels for the regions each species originates from for best measures.
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