You'll want the probe just above the substrate so that it is measuring the ambient temps of where the snake is spending its time. And side note, I have the same set up -- uth, CHEs -- with a 30 gallon tank. My lid is covered with plexiglass except for the two openings for the CHEs. Just fyi, adding the CHEs and getting your ambient up to 80 will destroy your humidity. I've tried everything to keep it in but the air is just too dry and cold. That being said, there was a good post by the user 'KMG' recently that talked about having the same issues and he just keeps moist moss in the hides and his snakes have clean, full sheds. So don't worry too much about your tank as a whole drying out. The snake spends 98% of its time in its hide so if the humidity is good in there it doesn't really matter what it is in the other 40 gallons. It's more important that your ambients are 78-83ish in order to prevent a respiratory infection.
