IT:No Vitals (Except the heart)?

[JCurwen3]

The heart is a more specific part of the vitals, listed in two places as being at -5 to hit because it's a smaller area (Horror, p. 30 and MH3: The Enemy, p. 10).

And Martial Arts, p. 137 makes an exception to allow for crushing attacks to target the vitals, mentioning the kidneys from the back and the solar plexus from the front (both abdominal vitals, at x1 wounding but shock causes a knockdown roll, at -5 if a major wound), and the implication is that this won't work for just any vital (it seems it's there to allow for kidney punches and the like).

If you use the more detailed hit location rules in Low-Tech which split the Torso into Chest and Abdomen hit locations, then it seems reasonable to rule that the random chance on a strike to the Abdomen to hit the vitals (1 on 1d) would only apply to the abdominal vitals (so never the heart - that would only make sense on a Chest strike), so using these more detailed rules your chances of hitting the heart randomly are also reduced.

So, given those considerations, what should the limitation reasonably be for Partial, Vitals, Except the heart for IT: No Vitals?

Bio-Tech, p. 172 gives a value of -60% for Partial, Vitals Groin only, and on p. 175 (Auxiliary Heart) it gives a -40% for Only to prevent first heart attack, if either of those helps as a rough gauge.