Approx 90 degree hot spot
Ideally use a heat mat with a thermostat
Your leopard gecko will be looking for a rather warm, small space to feel both toasty, and safe. You have multiple ways to heat your gecko, but most of the techniques suck.... Alright, well... obviously, that's subjective, but I'll defend my argument.
The first process generally involves using heat lamps, as many poorly written care guides will suggest. The issue is leopard geckos are not basking animals, and thrive much more nicely when they're able to simply lie down and absorb heat through their belly. This improves digestion, and keeps them comfortable. This is why using a heat mat is the superior choice with leopard geckos. Simply slide it beneath your enclosure, connect a thermostat, set it to your desired temperature, and you're all set. Although easy to avoid, this is unfortunately common when a heat source has no regulation. Find my thermostat and heat mat of choice with the supply link below.
90 degrees is the sweet spot for leopard geckos' warm area, or "hot spot", while temperatures can slowly drop the farther your gecko walks across the enclosure. The cool side will ideally drop to at least 80 degrees, but slightly cooler doesn't hurt!
This allows your animal to thermoregulate, or control his/her body temperature, as they are ectothermic (cold blooded).