1/3 to no more than 1/2 half of the tank should be covered underneath by the UTH. I use Zoomed mini size under one side of all of my snake 10 gallons. (The small size is too big IMHO.) They're the easiest to deal with, and the corn's like the belly heat. Try putting a piece of paper, or a tile, over the glass where the UTH is, and under the substrate so if the corn burrows, it won't get burned.