idk about milk snakes, but i try to keep Corny around 35%. Corn snakes don't really have a specific humidity requirement except for some misting during sheds, but i don't like it getting too low. tho i probably wouldn't go over 50% unless he's in shed.
with the sphagnum moss: start with a small amount and add as you need to. i don't think sphagnum moss will raise the humidity enough to cause any issues.