I was thinking about the great bonuses that a user of mentalism gets when wearing armor but no helmet, if there are no called shots. I remembered the GURPS system, which is quite simple.
Called shots, Adapted from GURPS, 3rd Ed., p. 203:
Head without helmet: -50 OB to hit; add +15 to the Critical Roll. Read critical results as if they were located in the head.
Head with helmet: -75 OB; add +20 to the Critical Roll. Read critical results as if they were located in the head.
Eyes: -90 OB; +30 to the Critical Roll.
Eyes through helm?s slit: -100; add +30 to the Critical Roll.
Arm, shield :-40, Damage over HPs? total/2 cripples arm. Excess damage is lost;
Hand, weapon arm: -40; Damage over HPs? total /3 cripples hand. Excess damage is lost.
Hand, shield arm: -80 OB; Damage over HPs? total /3 cripples hand. Excess damage is lost.
Legs: -20 OB; Damage over HPs? total /3 cripples hand. Excess damage is lost.
Vital organs (in torso): -40 OB; Impaling weapons that hit add +10 to the Critical Roll.