The cool and warm side gradient depends a lot on your room temps. If you have your snake in a room you can keep around 80, then you won't need another heat source on the cool side.
But if you, like me, keep your home colder then you may need a second heat source on the cool side and, of course, another thermostat to control it.

Otherwise, you place the heat tape on the bottom of the tub on one end covering 1/4-1/3 the tub. The helix probe goes right on the tape under the tub, and you'll set it, as was said, to whatever number gives you a steady 90-93 on that warm side spot. Use a reliable digital thermometer with probe to measure temps.