There are other methods to determine the sling length (L) that you may prefer, but for me I find the formula above comes in handy. Remember to rig safe!
Rigging tip: Did you notice that if you simply had measured the distance between the two pick points you would have had the same answer? Since 60-degree horizontal sling angle with COG in the middle equates to an equilateral triangle. (3 equal sides in length).