Let:

relativeEfficiency = distance between the weapon and armor reduction points
baseDmg = HP the weapon would take if it made a perfect strike
armorEfficiency = Number between 0.0 and 1.0 that reflects the efficiency of the armor
rnd = maximum ± randomness rate (so if this is 0.2, then the damage might change by something between 80% and 120%)
distance = the distance between the center of the blast and the affected ship
areaOfEffect = true or false depending on if the weapon has an area of effect (this is defined on the weapon)
random(x) = any random number in [-x; x]
Formula:
 if(areaOfEffect)
hp -= relativeEfficiency * baseDmg * armorEfficiency * random(rnd) / distance²
 else
hp -= relativeEfficiency * baseDmg * armorEfficiency * random(rnd)