I'd ditch the large water dish, horned frogs are terrestrial and a water dish that the frog can just fit in is fine. Forget about humidity, they're not fussy about it and aslong as the substrate is moist. I don't even use a hygrometer for mine. If the tank soil does dry then pour some water in and stir it up. If you're using an exo terra you can cover some of the screen cos they're over ventilated.