When I cook turkeys on the smoker, I always do this trick. Since my family knows what a smoked turkey looks like, I do not make a big production out of slicing it at the table. I filet the breast from the carcass and bone all the dark meat, including the legs. The times I have weighed the cooked meat from the carcass I have found that I usually get a 33% yield of equal amounts of dark and white meat. For example, a 15 pound bird will give you about 2.5 pounds each of white and dark meat.
I have done bone in breasts and due to the carcasses being butchered differently by different producers, the end result can vary from 30-40%. One other thing you might consider is cooking boneless skinless breasts. The yield on these is a lot higher since the only weight loss you have is from the water.