It's hard to say because chicken quarters really do have a lot of variance in size, but when I've measured before and after, 3 to 4 ounces has turned out to be average for breast meat.
To calculate my own, I would weigh my piece of cooked chicken, remove the skin and eat the meat off the bones. Then I would weigh the leavings (the skin and bones).
If you do this when you eat chicken at home, you'll get a better feel for how much meat a particular bone-in piece of chicken will likely yield.
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