Upper Chest
Low-Tech splits torso (9-11) into chest (9-10) and abdomen (11), which are not sublocations of torso, but are two separate hit locations, with chest becoming the default one. The coverage is 75% for the chest and 25% for the abdomen. However, it seems that area 9 is upper chest and area 10 is regular/lower chest, and those yet again seem to be separate locations, unless I'm missing some brain cells.
The coif from Low-Tech can be extended to cover shoulders and upper chest. A regular coif has 30% coverage, and the extended coif has 45% coverage. It says that it adds 50% (why only 50%?) of its DR to area 9 (but not the shoulders for some reason, even though they are explicitly mentioned above). As chest has coverage of 75%, the upper chest is 37.5% (more on that below). Since this coif provides 50% DR, I assume that these 37.5% are halved again for extra 18.75%. 30% + 18.75% = 48.75%, which presumably got rounded down to 45%.
Pectorals from Instant Armor protect the vitals (I assume only the chest vitals and not the abdomen vitals), and also partially protect upper chest (area 9). Now we also know that upper chest can be deliberately targeted at -1. The odds of protecting range from 1/6 (5% coverage) to 5/6 (16% coverage). For some reason, you cannot have armor that covers your entire upper chest, but if you extrapolate the coverage progression, you get the same 18.75% as from the extended coif above. Pectorals by default protect only from the front, so the full coverage for your hypothetical upper chest armor is 37.5%, which is half of 75% (chest). This means that chest is indeed split into two separate hit locations of equal size, but one of them is targeted at +0, and another one at -1. If you assign a -1 penalty to hit the regular/lower chest due to it having the same size as upper chest, you become unable to attack without a penalty (unless you attack a random body part?).
But why would you even target the upper chest specifically? There are only two kinds of armor mentioned that protect it and not lower chest, and there is no exclusively-lower-chest armor, unless you design it specifically. And since you target the entire chest by default, you can only hit upper chest either by striking it deliberately for some reason or when attacking a random hit location and rolling a 9.
If upper chest were a sublocation of chest, then you'd have to roll 1d upon hitting chest to see what you hit. "1" is already taken by vitals, and you cannot split 5 evenly between upper chest and lower chest. Thus, either you have to roll 1d twice - first to see what part of the chest you've hit, and second to see if you hit the vitals - or I am a complete knuckle dragger, or I am missing something here, or the rules are dumb. What the hell is going on here?
