Great questions. Terry answered about the humidity box. I use EcoEarth, and sphagnum works too as mentioned. Even potting soil works, I'm just afraid of what chemicals may be in potting soil.
How do you know when it is too moist? You may not, but the snake is no dummy. When given choices, they choose what they need. When the humidity here is 40%, my milks will hide all day in damp substrate, either over the heat (after feeding or when in a shed) or away from the heat. When humidity is moderate here, like 60-70%, the snakes can be found anywhere in the cage wet or dry. I also try to moisten some areas more than others. The area over the heat pad dries quickly, so it isn't always wet 100% of the time.
By doing the above, I never have seen those blisters that caresheets speak of, ever. I have raised baby prairie kings and milks that spent so much time buried in the substrate that I hardly saw them (except at night). They grew up healthy and fat.
Alot of folks use racks with little ventilation, and that can produce a nice moderate humidity without the need for humidity boxes. Depends on your ambient conditions of course.
Remember, these snakes live in the earth most of the time and gain their desired temps from the heated earth (heat mass). The above ground temps can be hot or cold, and depending on sun exposure they can still find desired temps in the soil, in burrows, under debris, etc.
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Mark