Greater minds than me will have a better answer, but essentially there is no rule. That is because the cooked weight depends on too many variables: the protein, the cut of protein, the cooking method, etc. As you have observed, there is a lot of fat in chicken thighs and it tends to render out when roasting. Breasts have less fat and would behave differently. And a chicken breast that has been overcooked will be dry and likely weigh less than the same breast cooked properly in a manner that retains the moisture.
That is why virtually everyone defines weights of proteins in their uncooked state.
Still, I'm not sure it matters all that much. I would think that the answer would range, at maximum, from 0-15%, perhaps 20%, with the fattiest cuts weighing in with the most weight loss.