Armor drop rates should be tweaked to match the need in a logical order.

Plate = 5 Classes
 
Cloth = 4 classes
Leather = 4 classes
Chain= 4 classes

So to determine further than plate being the most needed you would need to determine the server population for each other type of armor that have an equal amount of classes using it's respective type.

Having a +5% on plate drops wouldn't be a terrible idea, imo.