Anyway you slice it, it -can not- be more than 14,000 ft. And that is to Parthunaax. High Hrothgar couldn't be more than 11,000 feet or so, due to air thinning. 14k is about the maximum an unprotected human can survive in, and even then, unless you acclimated yourself, you would be very dizzy, prone to sleep, and not all there. Little things like breathing get in the way. And unless the dragons use magic to fly, 30,000 is far too high up for something that big to actually flap its way off the ground. Fall off the pinnacle in a controlled manner until it reached thicker air, sure. But a vto at 30,000ft? Not likely, unless the dragons weigh a fraction of what is implied by the booming landings.