I've been meaning to update this thread, with a full screenshot of my working model, for a long time!  As I often forget, and return here to remember how the Math_Function goes; because I really do'nt understand how it does work, still?

The goal has been to keep the Y-Transition, of the Plane(grayness,) constant; between the Preview and Render Windows.  Without the Math_Function, set properly, this just does'nt happen.  So we have achieved my goal; thank you all for this!

We've actually achieved my goal twice over.  The Math_Function node can be set to 1, .5; or 2, and 1.  In either case the plane stays put; but at 2, and 1 the Displacement Value causes more of a displacement?  In other words the White area of a map sticks up more, and the Black area down farther?

Lets review the Variables; and hopefully get a working Vocabulary?

There is the Displacement Value, which stops all displacement(black, white, and gray) if set to zero?  The Displacement Value could also be positive, or negative; in which case we get opposite displacement.

Then we have our Displacement Map values; White, Grey, and Black.  I think of Black as  -1, Gray as 0, and White as +1; which, somehow get Multiplied by the Displacement Value?   I use Multiplied loosely, because I ca'nt think of what does'nt happen when a negative Displacement Value is entered, and then multiplied by a Negative Black (my -1 hypothesis) which would thus yielding a positive Displacement?

Now enter the Math_Function's Value_1, and Value_2.  Why are there two values at work here, and/or what is their major contrast?  Does Value_1 just always need to be double what Value_2 is?

Am I missing anything terms?

One other thing I do'nt understand is the visual display, of the Math_Function Node; why is it that the White appears unchanged, while the Gray is no Black, and the Black is...well are we missing something there?