Yes I Do!
Glad you asked.
The severity of the crit is determined based on how badly injured that blow makes you; in other words, you never get decapitated unless you're killed by the damage (unless there are special magic factors in play or what have you).
The severity of the hit is determined vs. your current hit points when you get hit. Otherwise, characters with lots of hps will never suffer anything worse than a major wound. (For example, a Thrush, a 24th-level fighter, has a couple hundred hit points when fully healed, but it's been a long day and he currently is at 75. If he suffers a crit for 50 points of damage, he takes a grievous wound and the severity is determined by 2d12.
Roll for severity as follows:
Major Wound (less than half current hp): Roll1d12.
Grievous Wound (over ½ hp, still above 0): Roll 2d12
Incapacitating Wound (0 or less, still alive): Roll 4d8
Lethal Wound (dead): Roll 3d6+22.
Severity--Effect
1--Knocked Back 5’
2--Knocked Prone
3--Staggered 1 round
4--Dazed 1 round
5--Disarmed
6--Lose Shield (or off-hand weapon)
7--Break Finger, Toe or Nose; -1 to checks with hand/foot
8--Break Ribs (1d6); take 1d6 nonlethal damage whenever you run/charge
9--Lose 1d6 Teeth
10--Break Fingers (1d5) or Toes (1d5) or Lose an Ear
11--Break Hand, Foot, or Face; -2 to anything using injured part
12--Break Jaw; -2 to Bluff or Diplomacy, 20% verbal spell failure
13--Break Lower Arm, Lower Leg or Collarbone; -4 to anything using injured part, aggravating it deals 1d6 nonlethal damage
14--Damage to Armor, Shield or Weapon as well as target
15--Muscle Damage; take 1d4 Str damage
16--Break Upper Arm, Thigh or Sternum; -4 to anything using the injured part, aggravating it deals 1d6 lethal damage
17--Hamstrung, -10’ speed
18--Lose an Eye
19--Lose Fingers (1d5) or Toes (1d5) or Nose
20--Deafened
21--Lose Hand or Foot
22--Internal Damage; take 1d4 Con damage
23--Stunned 1d4 Rounds
24--Lose Lower Arm or Lower Leg
25--Lower Jaw Ripped Free; -8 to Diplomacy checks, 75% spell failure
26--Gutted; take 1d6 Str and Dex damage, 50% chance to tangle feet each square of movement, which inflicts 1d6 hp of damage to the victim
27--Vocal Cords Destroyed; no speech or verbal spellcasting
28--Brain Damage, 1d4 points each of Int, Wis and Cha drain
29--Knocked Unconscious 5d20 Minutes (healing equal to damage allows awakening)
30--Shattered Knee or Elbow; completely unusable, requires a regenerate to fix
31--Both Eyes Put Out, Blinded
32--Spine Shattered, Paralyzed (roll 1d6: 1– total; 2- neck down; 3- waist down; 4- left side; 5- right side; 6- arms only)
33--Throat Slit
34--Groin Destroyed
35--Heart Destroyed
36--Brain Destroyed
37--Decapitated
38--Unseemed from Nave to Crotch
39--Head Pulped
40--Cut in Half
My group has found this system to be a lot of fun. Everyone enjoys the chance to use a d12.