Simply clip in the Balance Clip on the arrow and move it until you find the point of balance.
Measure the distance in mm from the balance point to the arrow point (=S) and the overall arrow length (=L).
You can find more informations here.
With the Balance Clip not only you can determine the balance point of a single arrow, but also the difference in balance of a series of arrows.
Simply measure every balance point on each arrow of your series (dozen) with a pencil.
Align the arrows and you will see, if the lines you have marked on the shaft are all on the same height!