You actually want less bedding over the UTH, UTH's do not increase the ambient temperature so too much substrate could prevent the snake from easy access to the heat source. Also make sure you are not measuring the temps based on the surface temperature of the substrate, ball pythons like to burrow, the surface temp directly above the UTH (without substrate) is always going to be warmer than the temperature at the substrate level. If you are measuring your temps based on the substrate temperature you run the risk you burning your snake.