This is no different from any other skill. If you wait until the heat of the moment to practice a skill, you probably won't be able to do it under stress/strain. However, if you practice it regularly, it should be fairly accessible under stress. Where to shoot people is just like learning shooting discrimination. You can use the process taught by Paul Howe (Whole body, hand, hand, waist, face) and use it whenever you're out in public. Pretty soon you ability to rapidly assess is second nature.
Similarly, if you spend several months looking at folks and figure out where to place a round, it eventually doesn't require many mental resources. The quick and dirty method, for front facing targets, is to learn to visualize the spine. If you use it as an aiming point, you're rounds will naturally track where they need to be. Don't worry about slow, deliberate aimed shots, trying to transect the spine every time. Instead fire a "burst" of 2-4 rounds. Most of the time, at least one of those rounds will actually contact the spine and the ones that don't will have hit some important structures.
I also detest generic head zones. We should be using specific landmarks to place those rounds where they need to go. This is the FBI bullseye with a penciled in head zone.
Similarly, this is the USMC target with a bulleye added:
Both of those are "good enough."
If anyone can host a .pdf, I have this guy scaled up so you can add it to an IDPA or USPSA cardboard: