Tim,
Why 40 ?
-Irwin
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Mostly arbitrary, but 20 (ie 5%) doesn't seem to allow for sufficient differentiation in the bottom-most and top-most wedge. Once I've set brightness and contrast by eye to a 40 step wedge, I can see pretty well see one step gradations between 0 0 0 and 100,100,100 (actually the blacks aren't apparent until 98).
Going from memory of a method Michael described in his review a few years ago of the Sony Artisan, you can test the final results something like this...
Create a white 1000x1000 image (size is arbitrary) select the left half and turn off the marching ants. Create a curves layer (select pigment/ink% under curve display options) and select the bottom left point, using the up arrow you can move that point up one increment at a time. I can see the difference in left and right with output of 1 and input of 0. For the blacks fill the selection with black, invert the selection, hide the "ants" again. Drag the bottom left point up to the top and both halves will be black, (output 100, input 0) incrementally drop the output using the cursor and see at what point you can detect a difference between the 2 halves. If Pigment/ink% isn't selected just use the opposite end points.
This worked well for my Dell 1420 laptop. Of course what you can see in print is a whole different issue.