You might need more than a 40w for that height but I wouldn't worry too much about humidity, as I've said many times, horned frogs (accept from cornutas) are not tropical and so do not need high humidity but 60% suits most frogs well but if you can't reach this patches of sphagnum moss will help. If you're looking into another tank id go for a 15 gallon at least because I think most adult horned frogs - although males might be fine in a 10 gallon - females will be better in 15-20 gals