Damage to Armor

[vicky_molokh]

Originally Posted by McAllister

Actually, I... hm, you're right, we agree that the flat -1 is wrong. My mistake, I must have been spoiling for a fight when I posted.

To be honest, though, it may be more trouble than it's worth determining exactly how much DR each armor type has.

-1 seems like a simplification based on the assumption that armour has unimpressive amounts of HP and is Homogenous:

Originally Posted by B408

An attack only overpenetrates if its
basic damage exceeds the target’s
“cover DR.” To find this value, add
together the target or cover’s DR
[ . . . ]
and
HP (for flesh), 1/2 HP (for a machine,
vehicle, or other Unliving target), or
1/4 HP (for a Homogenous object).
Use the object’s DR alone if it’s a thin
slab, like a wall or a door. Finally,
apply any armor divisor.

Armour does seem to count as a thin slab, and either Homogenous or Diffuse. So at most ¼ of its HP contributes to its Cover DR; the rest of its Cover DR is in fact its innate DR.
Assuming that a single ceramic insert or whatever is about 1 lb of mass, that's a HP score of 8. ¼ that is 2.
Inaccurate, but easy to use in real time, in the middle of a combat.