Well, Black Milks do pretty well with a relatively high humidity compared to a few others, but to keeping it at 75% is a little higher than it needs to be, and will encourage bacteria and fungal growth within the enclosure. They will do just fine at a humidity of between 55-65% humidity. Additionally, Black Milks also thrive better at slightly cooler temps than many others too, being from the cooler, relatively higher elevations of the mountain regions of Panama, and Costa Rica. They are found at elevations ranging anywhere from 4300 to 7400 feet.
Regulating two temps in a very small container can be hard to do sometimes, so if you keep the young hatchling at between 74-79 degrees, which is usually a fairly comfortable temp for humans too, it will do just fine.
One strong word of advice though, put a RELIABLE thermometer directly down where the snake hides, as this is the temp that is important, guessing at temps does NOT work well. I would also put some more holes, or whatever it takes to get the humidity down just a bit to the numbers mentioned earlier,....damp paper towels tend to get nasty fairly quick too, taking those out just might get it closer to the target humidity also.
One last reminder, make ABSOLUTELY CERTAIN there is no small gaps, etc... where the lid meets the enclosure, and it is totally secure, or it will surely find it's way out. Ask anyone here how that works!..LOL!
good luck, ~Doug
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"Better to be silent and thought a fool, than to open mouth and remove any doubt!"