Approx 90 degree hot spot
Ideally use a heat mat with a thermostat
Your ball python will be looking for a rather warm, small space to feel both toasty, and safe. You have multiple ways to heat your python, the two most common are through a heat mat or UTH (un der tank heater) and a heat lamp/heat bulb. Some people use both if one doesn't generate enough heat.
Although there's no "right answer" with which option to use, keep in mind that ball pythons are nocturnal and generally want to stay out of sight, while still being able to warm up during the day. Naturally, ball pythons are from the hot climates of Africa, so even if they are away from direct sunlight, they're able to stay warm. I personally use a heat mat with all of my snakes, primarily because it makes it easy to deal with nighttime temperatures... since you don't want a bright heat lamp on at night.
Alright, now to the actual temperatures themselves. Based on their natural environment, and ball python breeder/keeper's success, approximately 90 degrees Fahrenheit (32.2C) is your goal hot spot in the enclosure. Some people go a bit warmer or cooler, ranging from 85 to 95 degrees depending on who you ask. This means the warmest accessible area in your animal's enclosure is at this temperature.
Attaching a reptile thermostat to your heat mat will ensure the temperature is exact, and will not result in burns on your animal. Although easy to avoid, this is unfortunately common when a heat source has no regulation. I set my ball python thermostats to about 93-94f, which in turn, leave the hot spot at about 90-91f.
Generally, it's best to put the warmest part of the enclosure near the far left or right, as this allows a temperature gradient from one side to the other. Basically - the farther you move from the hot spot, the cooler it gets, which is important for reptiles since they are cold-blooded. I feel best when the opposite side of the hot spot drops about 5-10 degrees. For example, if the warm side is 90, this cool side is 80 or 85. It doesn't need to be some exact level since your snake can move around the enclosure, but you don't want any overheating in your animal if the entire setup is the same, hot temperature.