The best way to work this out would be to duplicate the same algorithm Aaron uses in the damage calculation itself.
For leaky armor to work best it should represent the majority hitpoint contributor.
When designing "light armor" it should have a smaller size than the average component and provide a relatively high number of hitpoints. For "heavy armor", it should have the same or larger size than the average component, and provide a moderate amount of HP - but not too close to the corresponding light armor HP/kT ratio.
Keep in mind, light armor is more likely to allow a hit to break through and damage internals. Heavy armor will provide overall less protection, but will reduce the likelyhood of damage to internals for less total HP.