I would love to know the answer to this too.
What I did was to buy it from my butcher. But it was already cooked. So I started cracking the legs and put the meat on a scale but only put 3oz in stead of the 3.5 because I didn't know if the cooking would have cause a difference in the amount of meat.
I would love to know how others do it because I'm not sure it was correct.