If you want the histogram to tell you "dead on" expsoure information, you need a standard to judge the histogram by. Unfortunately, just aiming it out the window or somewhere in the studio won't work, since all of your shots will be different.
What you CAN do is purchase one of the folding three-tone targets (see the Wescott ads... there are several different models, and other manufacturers, as well). Or, you can check out some of the books made for photogs and Photoshop - mine was "The Phtographer's Guide to Photoshop" or somesuch (I loaned the book out so I can't check right now), and it had a target in the back of the book (like this one, with Kristen from the Dallas GG workshop this summer):
Unfortunately, the one from the book was glossy, and so it reflects light directly (look on the bottom of the black and grey targets).
Or, you can make your own, using a matte finished black piece of cardboard, an 18% grey target from Kodak or one of the others, and a matt white piece of cardboard.
With these black, white, and a 18% grey target, you can line up your expsoure so that the histogram indicates a proper expsoure for that target... put the spikes for those colors in the right place on the histogram, and now you have something that, with experience, will help you judge whether your expsoure is on or not.
Do you have an 18% grey card already?