The length from the wrist to the knuckle of the mid finger is just slightly more than the length from the same knuckle to the tip of the finger, i.e the knuckle of the mid finger divides the hand into two (almost) equal parts. The knuckles and the tips of the four fingers lie on an imaginary U-shape. The index finger just reaches the finger nail of the middle finger, the third finger is equal in length to that of the index finger and the little finger reaches the top knuckle of the third finger:
Watercolor Journal 