hmm it would be probly more useful under the heat mat, but depends really. you could have it on the cool side for a nice cool humid hide, or you oculd have it over the heat mat where the water will prettty much constantly condense and be nice and warm.
do you have a warm and a cool side...? usually red lights are used for heat, although a 2 1/2 foot tank is rather big, and if the only heat sources are the UTH and the IR light (infra-red/incandescent, red in other words lol), suppose it might be best they're at opposite ends. i tend to think of my UTH as the hot end. but its your call. try experiment, although it should probably be at the opposite end of your leo's current "dry" hide. it'll make him adventure and explore more ^^