What you really have to do is plot out the force draw curve for your bow to figure out how much energy the bow is storing. Then shoot through the chrony and weigh your arrows to find out how much energy is going into the arrow. Then you divide energy out by energy stored to get efficiency %.

Compounds store a lot more energy than their draw weight. Otherwise, I couldn' t get 62 pounds of energy out of my bow at 60 pounds draw weight.

And, like Bob said, efficiency will vary depending on arrow weight.