Nice to have the math behind the calculations. But, for most of us, it's very handy to have an online calculator for such things - especially when we have a number of rounds/caliber/gr. weights to play around with.
Regarding efficacy - these things can get a little subjective for sure. Hence, the ongoing debate concerning the efficacy of a high speed 9mm vs. a relatively slow speed 45, for instance. No doubt that the creator of this calculator comes down on one side rather than the other.
For sure, we all need to take into consideration such things as expansion, penetration, and the ultimate integrity of projectiles at the speeds our own carry gun is able to provide.
Regardless of the method/figures used - most will agree that frontal area of the projectile has "something" to do with it's effectivness in stopping the assault of a human assailant.
Witness the fact that the weapon of choice of dangerous big game guides is often a 12 guage shotgun with slugs or (applied at the last second of a charge) 00 buck that has expanded in patern no more than the size of a human hand.
It's proven effectivness in stopping the assault of dangerous soft skined game is legendary and without dispute. That effectiveness goes far beyond the mere paper energy of the round in question. The paper energy of a deer rifle often goes far beyond that of a 12 guage with slugs of buckshot.
But I know of no big game guide who would rely on their favorite deer rifle to stop the charge of a lion or leapard at common pistol fighting ranges.
But the purpose of the post was not to argue his effacacy conclusions via a formula. It was to provide a very handy tool for ammo selection thoughts and for playing around with such concepts in general.