[Anthony]

Note that a difference of squares model isn't really accurate anyway; the amount of energy that is required to breach armor may not be the same as the amount of energy absorbed by successfully breaching armor. If you were to make a table of projectile energy vs energy absorbed, the difference of squares model implies that it would rise up to the point of penetration and the turn flat, but what it's typically going to do is rise up to the point of marginal penetration, have a bump of some sort, likely dip back down below the initial peak, and then gradually start rising again.

All this is why armor models have never been very concerned with the effectiveness of armor at reducing damage that penetrates, they're far more concerned with what is stopped outright.