You are gonna need to set it to where it allows you to achieve proper temps and depending on how stable your room temp is you might need to adjust it daily.
Really?that is why you should NEVER use ANY under tank type heating element with some kind of thermostat or rheostat. you can get the stuff to make dimmer type unit for under twenty bucks.
A thermostat is the single most important piece of equipment, cutting corners is not only dangerous for your animal but can also become a fire hazard.