Went over the backside of the hand with more annotatations. A bit more structure to how I was labeling the measurements totday, Broke things down into units of W, X, and Y.
* W being the width of the wrist (to act as an anchor to proportions of everything else later, like how the head is for the rest of the body)
* W/2 is the size of the wristbones, approximated to half of W
* X is the length of the knuckle segments on everything except the tip of the pinky and middle of the middle fingers
* W/2 is approximately the same as X
* Y is the width of a finger
* Y is also approximately X/2

Then some more measurement observations
* First knuckle of thumb is 1W=2X from the inner wristbone (offset at the same angle as the direction of the thumb)
* top of palm/start of fingers/knuckles is ~1.5W=3X
* index and ring fingers are roughly 3x (same as height of palm) - the two can have slight variation between whether one is slightly longer, but never more than the middle)
* middle finger: ??? my own is way off from the ref so idk. In the ref, base and tip are X, and mid segment is X+Y, but on mine, middle segment is X, but tip and base are X+Y/2 (takeaway: middle finger is roughly Y longer than the ring/index)
* pinky is roughly X shorter than ring. Not entirely sure where it shaves off since it seems roughly aligned (base segment and middle segment maybe Y/2 shorter than ring) -- either way, tip of pinky should align with last knuckle of ring
* nails start at X/2=Y on the final segment

Please excuse the algebra for relative proportions. My day job is engineering, so I think/conceptualize better in math terms than abstract/non-geometric shapes (I suck at extrapolating "this kinda triangle with an arc is the palm pad" stuff doesn't work as well for me unless I can get enough mental measurements
I'm going to continue this measurement breakdown in the other poses, just to make sure it stays logically consistent with the rest, then I'll try to build the geometry