Don't, it's rather useless and just a marketing driven 'feature'. Control the ambient light, don't adjust for it in the wrong place. IMHO, this has nothing to do with your issue.
If your ambient light is bright, your display needs to be brighter, if the ambient light is dark, your monitor needs to be less powered in backlight. And what would be the right brightness and contrast for each ambient light situation?
For me it helps to have it adjusted automatically one I keep my ambient light quite the same controlling it as you say. Otherwise, where's the reference?
Thanks for the replies. Controlling the ambient light has been my routine for years, so that confirmation is helpful. I try to standardize both the display brightness and the light levels in my work area (easy at home, sometimes more work when traveling, but seldom a problem).
As a point that's only partly off topic, I always make sure that my notebooks have all of the automatic brightness adjustments turned off, too. One of my recent Dells had a particularly annoying ambient lighting automatic adjustment that could be turned off, but had to be turned off in the advanced power settings for each of the power profiles that were set up in the operating system. Grrrrrr....