Kevin, I think that's a reasonable rule of thumb. I personally think the "10 gallon" rule is not always appropriate (every care sheet on horned frogs says that you need a 10 gallon tank at minimum for a single adult horned frog) - your typical 3 inch male cranwell's is actually kind of lost in a 10 gallon tank. A cautionary note, however - the bigger the tank, the more leeway you have in terms of cleaning. If you keep a horned frog in a tank that is only 6 to 8 times the frogs "area", you must stay on top of cleaning or you really run the risk of bacterial ("red-leg") issues. Avoiding over-feeding your [adult] frog helps a lot (less in, less out).