I'm going to propose that Plate armor is the best defensive armor in the game. Most armors probably do not reduce damage by a substantial amount, even for those damage sources they are optimal against. Most, that is, except Plate. Like most armors, it doesn't help much against other damage types, but the one type it does defend against is reduced by a ton.
Put simply:
Defensive utility of all other armors for perfect damage sources: ~0
Defensive utility of all other armors for imperfect damage sources: 0
Defensive utility of Plate armor for all non-normal damage sources: 0
Defensive utility of Plate armor for split damage sources: ~0
Defensive utility of Plate armor for pure normal damage sources: 1
Plate armor wins.
A "perfect" damage source would be, say, a Slag Walker swipe (normal+shadow+fire) versus Fallen armor (normal+shadow+fire). An "imperfect" damage source would be, say, a gun puppy bullet (pure elemental) or slime tackle (normal+pierce) versus Vog Cub Coat.
All this is just a theory I've been cooking up. Can anyone confirm or disprove it?
I don't know the answer to your question, but your analysis seems to be based on a rather imprecise quantification of how armor protects knights. There have been studies of how defense works, such as here and here, that may help you refine your argument.