I follow the basic guidelines well vetted by common use among many breeder/keepers, but with a few caveats.
The three temperatures, 1) day time cool side, 2) day time warm side, 3)night time ambient. Some people advocate for a ‘basking spot’ as a fourth temperature. I prefer to have a range maintained by a ‘hot zone’ rather than a basking spot. Basking spots have the potential to desiccate the keratin on the shell leading to shell malformation.
A hot zone can be created with a low wattage Ceramic Heat Emitter (CHE), or a heat panel. The panel should be horizontal and at about 1.5 to 2 inches over the shell height off the top of the shell surface, the thermostat for it should be placed so it is at the same level as the top of the shell, set for the highest temperature you seek, I suggest 95 to 98 F. If you use a CHE I suggest directing it to a ceramic floor tile, that is placed above the ‘hot zone’ in the same height as described for a heat panel. DON’T place the CHE directly on the tile, but about one inch away. Again, place the thermostat controlling the CHE at shell height under the tile.
This manner of heat generation makes a ‘hot zone’ that will not dry out the shell as would a direct application of a CHE or any kind of light.
The cool end can be made warmer, if needed, with an additional heat source such as a seedling heat mat affixed to the side of the enclosure, not the bottom.