Kambar is right.
For a character turn-around, standard tricks like the head-method may work, but it's pretty limiting. Comparative measurement and careful observation beats a memorized formula any day, and it teaches you observational skills which will be useful for drawing completely unrelated things as well! I suggest comparing angles and distances and looking at negative space in proportion to other parts of the same image (assuming you are working from a reference or life).
Of course, the easiest advice I can give you is: Draw longer legs! No, seriously. Whenever you draw legs, force yourself to draw them longer than you normally would, even if it feels awkward at first. Eventually you will make a habit of it.
'Cuz life is full of your regrets, and I should be one...