You always want to make sure the glass temp is what you are reading. Should be about 89-91. This way you know they are never going to get burned. I put down less substrate over the UTH side so the snake can reach the heat easily. My snake always pushes away the thin layer of substrate over the UTH to reach the heat.
Even then the snake may not use the hot side too much. When my ambient temp goes up my snake usually stays on the cool side until a meal.