Omegonian is correct, you used the rule wrong. He used that rule to figure out how his massive materials like Neutronium deal damage if they get somehow turned into weapons and wielded, it was never intended to be a global blanket rule you just plain apply to damage calculations.
Think about it this way: if you double the energy, it's essentially the same thing as if you took the single energy event, and did it twice. What your "derived rule" essentially says, then, is that you need to make 64 strikes to deal the damage of two hits. To me, that's patently absurd.
I maintain that the estimate of 3 x 10^30 hit points is more correct for a planet, unless perhaps you're using severe four-color-comic-book physics and some sort of exponential damage system. D&D does not use an exponential damage system, though; it uses hit points which are brought down one by one (even if a single attack can bring down more than one at a time); the system is as linear as it gets.