Blender Displacement Node Domesticated: the actually getting it right this time.
So I made a post a little while back being all triumphant and "I've cracked the code for displacement!", crowing that if you run the height map input through a divide node you can easily get a predictable displacement distance.
And it does, BUT only if your displacement node Midlevel is set to 0.0 (left image). However if it's set to anything else, like the default 0.5, the whole mesh gets turned inside out! (right image)
Buuuuuut it turns out I was very close, the whole "the height map is measured in Blender units" thing is right, but this is what the SCALE control is for! It's doing the whole "divide the height map" for me only not broken!
So if I unplug that [divide by 10] node and instead set the set the scale to 0.1, then we get the same 100cm deviation on the midlevel 0.0 image, and a 50cm deviation on the midlevel 0.5 image BUT Suzanne isn't turned inside out!
So yeah...
TLDR: Assuming you have a height map with values from zero to one, you can have millimetre precision on displacements by plugging a [divide by 1000] node into the displacement scale, then just entering how many total mm you want into that!