Side note: Rigorously following the formula can make you a little predictable, if your opponent knows what it is: It's helpful to prioritize your enchantments, and make the first-cast one, the one you put the most gems into, the one you REALLY want to stick.....then follow by throwing up a decoy enchantment in a few turns, one which he will probably find more distressing, and then let him tear it down following my formula, depleting all of his energies on a decoy.
Also, when in doubt, double the numbers. Anything worth doing, is worth doing with excessive force. Admittedly, the formula I've given *ALREADY* includes an excessive force multiplier, but like I said: If it's worth doing, it's worth doing with *EXCESSIVE* force. Obviously, if excessiveness can be so easily quantified, it's no longer truly excessive.