Creative Image Editing in Photoshop - Black and White in Photoshop CS4 and Photoshop Lightroom - page 209

Save these as your target values!!!
Click OK to the Color Picker settings, and again to the Levels box, and “Yes” to
save the new values as the default settings.
Setting black and white Points using Levels
We will outline two methods to determine black and white points within an
image. Method 1 is a faster and easier approach, while Method 2 will prove
distinctively more accurate in determining point placement, but also a bit
more complex. The process for setting black and white points for color and
black and white images however is exactly the same utilizing either method.
Method 1: This Method is Easier!
After the target values for black and white are set in the Eyedropper tools.
(See “Changing the Dropper Default settings”, page 208.)
Step 1: Create a Levels adjustment layer
Step 2: Option key sliding
While holding down the Option key (Alt key on the PC), click on the white
point slider in the Levels display box (just under the histogram). The image
will turn completely black. As you pull the slider, white will eventually begin
to appear within the image indicating where to i nd the lightest area that
contains detail within the image. This is very helpful when deciding where to
click the eyedropper. You will be looking for the i rst place white presents itself