I'd go for 85-88 degress for the warm side, but Corns are pretty tolerant. In temps of a steady 80, I'd still be inclined to provide an extra warm spot as suggested by Danielle. Ideally they need to thermoregulate (regulate their body temp by moving between warmer and cooler areas) and a warmer spot helps with digestion.
Having said that, there are folks here who have dedicated reptile rooms where the room itself is heated to one uniform temp with nothing extra in the tanks.
Nothing's straightforward with Corns!