Quote - ...if you were to push the bump depth up quite a bit, for whatever reason, you'd start to see artifacts around the folds of the eyelids unless you have the subtract .5 in there. When people use a bump depth of 1 millionth of an inch, it doesn't matter much that mid gray is actually the zero-point, and you're getting two millionths of an inch of bump. However, if you increase the bump 5/100ths of an inch, and then you double it because of the Poser Black=0 phenomenon, well you'd see the problem. Then you'd wonder why my shader doesn't show artifacts and yours does....

Adding or subtracting two numbers is a concept we are familiar with. Even adding salt or water to soup makes sense. These are paradigms that are familiar to us, and so we have a frame of reference to work from.
Colours? Sure adding red and yellow gives us orange... but I think most of us have pretty much arrested at that stage. When I say 'us', I'm not referring to this group, who obviously have at least partially your feel for things material-roomish... it's the rest of us Dummies who so want to get a feel for it, but are daunted by algebraic formulas and Poser Black phenomena. This is why I'm biting the bullet and doing this wee lab... but perhaps I'm leaving out an important component, aren't I?: understanding the maths behind it. I kinda thought seeing the physical changes would suffice. I can see, however,  that because of those aspects of the material room that can only be appreciated mathematically and that profoundly affect the final render, I'm probably wrong.

Monterey/Mint21.x/Win10 - Blender3.x - PP11.3(cm) - Musescore3.6.2

Wir sind gewohnt, daß die Menschen verhöhnen was sie nicht verstehen
[it is clear that humans have contempt for that which they do not understand] 

Metaphor of Chooks