@InspectorCaracal i'd say maybe look at ref sheets used by animators - that kinda gives you a good baseline.
personally my starting point (if i had spoons to do it up proper) would be
1. full body facing forward
2. full body behind
3. head closeup, profile
4. head closeup, head-on (or 3/4 if you're feeling sassy)
@wigglytuffitout *scribbles down a whole bunch of notes* thank youuu
@InspectorCaracal the main goodies i've seen be extremely useful are annotations (scar x, marking y, always wears necklace with additional closeup of it, etc), and colors off to the side in blocks (easily eyedropperable). so a list like - eyes [block of color], fur [block of color], separate from the drawings. even putting a hex code of the specific colors if you REALLY wanna be fancypants and make life easy on artists later commissioned.