Having a go expressing this:

Quote - ...combining the two, such as Gain(Bias(x, .8), .8) would give you a sharper edge, closer to the outside of the transition zone. And Gain(Bias(x, .2), .8) would give you sharper edge, closer to the inside of the transition zone...

...in nodal form, but I'll have to admit: even after studying your Matmatic tutorial and the SimpleShader script, I still have nil clue what to connect to what, based on those formulas.  I'm figuring that all I want to affect is just the lips, so where the maths need to happen is just to the Mask, but how to connect the nodes up is way beyond me... been struggling with this all morning.

Also noticed that when I do a synchronize, my renamed VSS image map node gets duplicated in the final figure, so that I've ended up with multiple instances of, say, a Lips_Mask node, with the last one I created actually connected. I'm sure I'm stuffing up, here, somewhere... both the template and material shader have the same names for that particular node.

Okay, back to the SimpleShader Scripts... the secret is in here, somewhere. Might just run each script and see what makes what...

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