Lacey, I use those measuring techniques only when I can't resolve the measurements by eye. If you read Tony Ryder's book, the text gives a very convincing argument for measuring by eye alone. To be honest, I can't remember how he explained it, and I don't have the book handy, but I remember that he made a very good case for trusting (and training) your eye. My work improved as a result of studying his book.
There are two main approaches to catching a likeness. The Academic approach, with "sight-size" and strict measuring, can often result in a photographic likeness, even when working from life (sorry, Hockney). Then there's the approach that Sharon mentioned, which can capture the likeness in a more gestural manner. Both are valid. (
See Peggy's drawing of her son, which was done from a photo. I love that drawing, and it in no way looks like a copied photo. )
As for working from photos, I'm considering photoshopping an image out of focus, as a kind of virtual squint, to force myself to ignore details. Then, when it's time to add details, I'll pull out the focused image (or not!)