Firstly: that IS a practical value. Do you know the proportion of an arm that a human's hand takes up? You can sort of eyeball it, say that sorta looks about right, but the best way would be to measure it or:
Secondly: I think the best way to use an image is to trace over it with your points. If you're plotting the points, just follow the lines of the reference. Even for something like box modelling: scale / extrude the points so they follow the reference image.
I really think reference images are essential for making realistic models of things that are difficult to imagine (even the human body, which you probably see every day). if you want something to look real, you really need a reference.