That humidity isn’t too bad for when the snake is not in shed. I’d suggest covering 2/3 or more of the screen top with something to minimize moisture and heat loss though, especially during the colder months. Aluminum foil/plexiglass/etc will likely help your humidity levels. I try to aim for 65-70% or so when in shed.
You can use the dimmer lamp with a thermostat no problem. Thermostat will just adjust temps for you rather than having to fuss with the dimmer all the time.
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