The curator at reptile gardens has a weird way of keeping snakes and I am going to try it out. His philosophy is that in captivity snakes don't need to thermoregulate as they do in the wild. He says every snake he has worked with likes to and can do everything they need with a core body temp of 80ish so he doesn't use a hot spot for anything and just keeps ambient temps at 78-82. If you can buy in to that philosophy your bull snakes should be just fine at that temp.