i would have said it was

chance to kill = accuracy*shield*(armour+armour piercing ability)*(0.18*lethality)

where armour = (1 no armour .9 some .5 loads etc.)

armour piercing = (0 - 0.5) but when applied makes a maximum of 1.

Armour really sholdn't degrade when hit, the chances of hitting the same spot twice are tiny, and even if you did, it's a proven arrow deflecting spot. Also if the origional arrow was lodged there, it has even less chance of working.

form my games workshopping days, something of the order - roll to hit, roll to damage, on those hits that damage roll to armour save with modifiers applied to natural armour save.

of course i don't really know just makes sense to me like that.