Where are you getting these formulas from? Sounds to me like you're kind of guessing with a few of the numbers. How do you know the reduce isnt before armor?? eg) 15dmg -2 x whatever your number was. In which case the -2 would be much less insignificant.
edit: let me elaborate
Knight: 80 AL = 70.7%, 30 * .707 = 21.21 - 2 = 19.21
Plate: 85 AL = 64.8%, 30 * 64.8 = 19.44
your numbers
now lets see my theory (which I think is more logical according to balance reasons)
we'll do 30 damage again?
Knight: 80AL = 70.7%, 30-2 (.707) = 19.796
Plate: 85AL = 64.8%, 30 x 64.8 = 19.44
Wouldn't that make more sense?
edit 2: physical re
take into account I'm trusting all your damage reduce based on armor (I dont know the formulas
With Knight: 30 * (.595) = 17.7 -2 = 15.2 !
With Plate: 30 * (.545) = 16.35
your numbers
and my theory
With Knight: 30-2 * (.595) = 16.66
With Plate: 30 * (.545) = 16.35
So conclusively, shouldn't my math make more sense based on the cost of the armor? Still we see little difference and I'm aware of this but the difference is still apparent. However as stated above you can have the best of both worlds and recieve the damage reduce bonus globally based on 1 part and assuming it doesnt hit that part of armor you're going to look like this:
With Plate: 30-2 * (.545) = 15.26 <<clearly the best solution
Although after looking at this I just realized that my chest/legs are ascalon armor, I need to change that

I bought thinking expensive = better.