@MissNancy
Ah, never knew that!

And that creates a different issue then; like how do vendors go about ensuring that when a user applies a material preset, the displacement will have the correct amplitude?

@Paolo
ZBrush has a function called "Mid" in the Displacement Map panel, I think that's what you're getting at.  It's a slider which lets you set the mid-point of the displacement amplitude.  It's at 0.5 by default meaning that mid-grey equals no displacement, but it can be adjusted by sliding the control between 0 and 1.

At least I think that's how it works, I'm only just getting into displacement :-P