Digital BW, The Print

Any suggestions on how I can come
> up with a set of "base-line" numbers or values for each of the steps on a 21
> step grayscale?

Craig,

those "base-line" numbers are also known as "linearization" and there are 
several ways to approach that, but none may be practical for your purposes:

- with devices that put down dots or haftones, common prepress practice is to 
consider that 50% means that in a given area (haftone dot) there are equal 
parts paper white and black ink. Other percents follow that logic until at 100% 
there is only solid ink. Software translates these from density numbers to dot 
percents based on the density of the pure black and the pure white. Dot gain 
is then also calculated.

- with continuous tone devices, a manufacturer may provide a set of ideal 
densities for each step, based - again - on the density of the max black and 
white. 

- in all cases, the transition from white to black has a slope, known as gamma, 
and obviously there are many ways to lay down these steps, based on your 
chosen gamma. If you can lay down a grayscale that matches a working gray 
space in photoshop - such as 1.8 or, more likely, 2.2 - you'll have an easier 
time going from scan to monitor view to print. That's the route that ImagePrint 
has taken, as an example.

I know it's all a little theoretical, but maybe it can point to a solution for you.

Antonis