Take one probe and put it right on the UTH inside the tank (right on the glass under the substrate). This temperature should always be below 95*F. Shoot for 92-94*F. You absolutely need a reptile thermoSTAT (reptitemp 500r will probably do you well) to safely use a UTH on a tank.
Take your other probe and put it on top of the substrate by the cool side hide. This is your cool side temperature. It should be between 80-85*F. Don't let this temp drop below 75*F.
I really doubt you need the bulb if you said the temp goes to 80* without it. 80* ambient temp is just fine.
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