I realize this thread is old, but I had to throw this out there.
I read somewhere Sargent began many of his portraits by measuring his subjects' head with calipers and subtracting 1/4 inch. I don't remember where I read it, but I remember it specifically because I thought it was such a good idea.
Also, using the head as a measuring tool began many, many years ago... Although, one could as easily use a foot, or a forearm, anything that could be compared against something else. One of my teachers taught us, when drawing, to start with the thing closest to us (lower leg, for example) and then use the size of that to measure everything else against. I've always been a head measurer, though. His method is much better for creating the illusion of depth and foreshortening, I admit, and I should use it more. Old habits are hard to break... I'll have to pay more attention.
With regard to the relativity of our body parts... did you know the length of your nose is the same as the distance between the first and second knuckle on your index finger?
Or, that a person's height is usually equal to their arm span?
Leonardo da Vinci wrote much on the proportions of man and the perfection of the design, using it as an argument as to how to design architecture:
This is where Sharon's illustrators' book "eight heads guideline" originated. (Well, I don't think da Vinci originated the idea, as he refers to the ancients, but he did, most famously, write it down.
Vitruvius, De Architectura:
THE PLANNING OF TEMPLES
--snip--
2. For Nature has so planned the human body that the face from the chin to the top of the forehead and the roots of the hair is a tenth part; also the palm of the hand from the wrist to the top of the middle finger is as much; the head from the chin to the crown, an eighth part; from the top of the breast with the bottom of the neck to the roots of the hair, a sixth part; from the middle of the breast to the crown, a fourth part; a third part of the height of the face is from the bottom of the chin to the bottom of the nostrils; the nose from the bottom of the nostrils to the line between the brows, as much; from that line to the roots of the hair, the forehead is given as the third part. The foot is a sixth of the height of the body; the cubit a quarter, the breast also a quarter. The other limbs also have their own proportionate measurements. And by using these, ancient painters and famous sculptors have attained great and unbounded distinction.
3. In like fashion the members of temples ought to have dimensions of their several parts answering suitably to the general sum of their whole magnitude. Now the navel is naturally the exact centre of the body. For if a man lies on his back with hands and feet outspread, and the centre of a circle is placed on his navel, his figure and toes will be touched by the circumference. Also a square will be found described within the figure, in the same way as a round figure is produced. For if we measure from the sole of the foot to the top of the head, and apply the measure to the outstretched hands, the breadth will be found equal to the height, just like sites which are squared by rule.
--snip--
Book 3, c. I1
More here...