Sorry Lija but horned frogs are very hardy when it comes to humidity and I seriously doubt they'd die because of that. They have adapted to dry conditions in their natural semi-arid habitat by burrowing. They're not tropical like dart frogs I don't even bother measuring my frogs humidity anymore as they're so hardy and just aim to keep the soil moist, as when they are active it's during the rainy season but when the tank has dried, the frog will just burrow deeper for moisture. Most frogs however, will do fine with at least 60% humidity.

A ceramic heat emitter or heat lamp in a 2.5 gal will most likely turn it into an oven and I'd only use a heat mat in such a small tank, to give a good thermal gradient. Ten gallon is best for a starter tank and then going up in size if you have a female. Covering 2/3 is far too much and should only cover half, if you even bother. I do agree if you're using a ceramic or heat lamp, it might be best to invest in some sort of fogger system to prevent the frog drying out.