I think you will find an easier setup by adjusting the wattage of the bulb, and changing the distance from your basking spot. Closer for higher temps, further away for cooler temps.
It can be tricky to setup a thermostat probe right in the basking spot, and if it gets covered by foliage, or by an animal, the thermo then fires up the bulb more to try and compensate, making it even hotter.