If your warm and cool side temps are appropriate you do not need any belly heat. Take all heat measurements at the level of where the snake lives. Snakes don't breathe the air that is at the top or middle of the tank. Most large PVC enclosures for boas/retics/burms use RHPs as the sole source of heat. Think nature. Sun warms the earth, snake soaks up warmth. As long as the microenvironment around the animal allows thermoregulation you are in good shape.