This method requires an extra step or two, but is useful if you have a difficult image (like Russ has with the 498 photo)
In Photoshop (and Elements too) there is a tool called Threshold. It’s found in the Adjustments palette, also in the Image/Adjustments/Threshold menu. By opening an image you just scanned, (or any image for that matter) in Photoshop and then using the Threshold tool to evaluate it, you can precisely find the white and black points. A histogram of the image is shown when you open the tool. Right away you can see if your scanner is in the ballpark. If the histogram isn't almost touching both sides of the 1-255 scale, your scanner controls should be adjusted. (If it is touching the sides, the scanner is doing all it can in that direction and the shadows may be too dense or the highlights overexposed.) The slider along the bottom is set at the midpoint (128) when the tool opens. By moving it left and right and watching where either the white or black completely disappears. That is the value (1-255) where your darkest shadow or brightest highlight resides. It also shows you exactly where that point is located in the image. If your scan software allows you to use the 1-255 scale you can set the numbers from Threshold and be right on or very close.
Be aware the threshold tool works with what is called a luminosity histogram. See this link for a technical explanation of why that is important to know: [
www.cambridgeincolour.com]
In short, individual color channels (red/blue/green) can experience clipping/color shift if the image contains very saturated colors. We are just trying to get it close, so the Threshold tool still works in most situations.