RN, 17 years, some OR time, some radiology time, tutored human anatomy.
I am shooting for the ribcage and carrying enough gun (.45Colt, 255gr at ~950fps) to shoot through an arm into the chest if I need to.
I agree that liver wounds bleed a lot, but they bleed -relatively- slowly because the only place for the blood to go is out the entrance and possible exit wounds. Even a single lung shot will collapse that lung and provide space for the blood to flow into, faster bleed, faster incapacitation. On a head on shot a bullet that misses the heart, great vessels and spine will likely get one lung if it hits the BG at all.
When I visualize a bladed target I am visualizing center of ribcage. If I miss the spine, and the aorta and the vena cava and the heart I am still going to get a second lung at any likely engagement distance.
Really good question though. I should practice some with an IDPA target twisted to an angle sometimes.
Thinking about it I have avoided practicing on oblique targets because shooting someone in the back hardly seems like self defense, but I need to think about this some. I pray I never have to do it, but worse case (mebbe) I shoot at a bladed assailant and miss the ribcage. Entry wound on one side of the back, bullet tunnels under the skin, smacks a dorsal process on a thoracic vertebra hard enough to stun the assailant long enough to end the fight, then the BG wakes up paralyzed from wherever down and I get to go to court.
Clearly not shot "in the back" but from the side. Morally defensible, but I got to work on my speed getting out the second half of a controlled pair to the ribcage.