The "Ambient Temp" of the air inside the tank is not too critical. your snake does not fly through the air nor does he stick to the sides. He probably lays or crawls along the bottom like most snakes do. That's where you measure the temps - where he actually is.
If you have 85 degrees on the warm side and about 70 to 75 on the cool side on the bottom then that's perfect - forget this "ambient crap." The extra heat and the overhead lights and heaters and so forth will only dry out the air and cause more problems later on.
Karl, maybe you don't mean to come across like a jerk but when you talk down to people, well, you do! Throw a smilie in there if it's your attempt at humor so the OP knows
As for the "ambient crap" (AMBIENT ADJECTIVE 1.of or relating to the immediate surroundings of something) It is ALWAYS about ambient temps. The most important is surrounding the snake where it is. You are right that ambient temps anywhere off the surface is not important. Maybe you meant to say that general air temps "room temp" are not important. Getting the right ambient temp next to the snakes skin is wherever he/she is which tends to be in the warm hide. You are measuring ambient temps with all probe thermometers unlike a temp gun which measures the temp of the actual object.
I'm always correcting the usage of the term "requires belly heat". I think "bottom heat efficiency" or something like that would be a more accurate term as corn snakes do not need to heat their bellies directly in order to digest. A corn snake will seek out belly heat but in relationship to health and digestion it is unnecessary.
The notion that UTH's don't dry out the air is off base. They just don't do it as quickly as an overhead heat source. I think to many people are rigid in how a corn snake habitat should be setup making no allowance to certain situations. Sometimes in certain setups an added heat source is necessary and even though a OTH is least desirable sometimes it is what works.
OP, Karl is right that if you have 68-70 on the surface of the cool side and 82-85 inside the warm side hide without the CHE then your good. If
your room temp is so low as to lower those two temp reading then go ahead and continue with the CHE
Have fun snaking :cheers: