Approx 85 degree hot spot
Use a heat mat or heat lamp
Achieving the correct temperatures for corn snakes isn't too difficult, as they don't need ridiculously warm hot-spots. You also have multiple ways to heat your corn snake, 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 corn snakes will generally want to stay out of sight, while still being able to warm up. Naturally, a colubrid will either bask out on top of something warm, like a rock, that's been heated up by the sun, and in other cases, they will hide beneath objects that have warmed up, like tin or wood. I personally use a heat mat with my corn snake, 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 corn snake breeder's successes, approximately 85 degrees Fahrenheit (29.4C) is your goal hot spot in the enclosure. This means the warmest accessible area in your animal's enclosure is at this temperature.
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 85, this cool side is 75 or 80. 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.