An alternate way to set the black and white points is to use the Eyedropper tool. I prefer to use the histogram myself, but this tool should be explained.
This is Umax MagicScan 4.2, and others work the same way, the eyedropper is a very standard tool. There are two eyedropper tools on the left panel, a white one and a black one, for highlights and shadows. Selecting one changes the mouse cursor to be an eyedropper symbol. Then, moving the mouse over the preview shows the color value of the pixel under the cursor in the Info Window.
You can see the black eyedropper cursor in the lower right corner of this preview image, on the edge of that dark leaf (this is not the darkest spot on this image, but the cursor shows up well here). You would look for the darkest area using the black eyedropper. Is it the lowest leaves? Or the center of the flower? Check it out, the eyedropper will tell you. Similarly, you look for the lightest area with the white eyedropper, the upper left corner of this one.
The Info Window is a magnified view of a few pixels around the mouse cursor, and the one pixel selected (under the mouse cursor) is shown as the white square in center of the Info Window. The RGB color values of that pixel are shown too. Now, the point is, if you click there, that color value will be set to be the Black Point, the same as if you selected that tone value in the Histogram. Same for the White Eyedropper, using it to click in the Preview window will set the White Point to that value. This method sets the Black (0) and white (255) points to be at the extremes that you specify this way, using the pictorial image to judge your preference, instead of the histogram data.
I personally prefer to use the histogram, but opinions do differ, and the eyedropper is a very standard tool. Both methods work and achieve the same purpose to correct the contrast of the image. In both cases, according to your particular criteria, the correction is implemented by setting the Black Point (the point that will be black) to a specific value, and setting the White Point (the point that will be white on the grayscale) to a specific value. The rest of the tones will be spread proportionally between these end points.
The Microtek ScanWizard eyedropper is on the Preview toolbar. "Click" is the Shadow, and Shift Click is the Highlight Point. You can specify one RGB channel if desired. The tiny button on the Info Window can specify the size of the sampled average, to be 1x1 to 5x5 pixels.