Actually, you can take any percentage less than 50% and throw it right out the window, because genes don't work that way. Genes are packaged into chromosomes. Chromosomes come in pairs; one member of the pair came from the mother, one from the father. Sometimes, chromosomes in a pair may "swap" parts with each other, but most of the time, chromosomes get inherited intact. A chicken has 78 chromosomes. Even in a cross between two purebreds, slightly more than half of the genetic material may have come from a bird of one breed, slightly less from the other (because the pair of chromosomes that determine gender are very different sizes). When you breed a 50/50 hybrid, you have no idea how many chromosomes from which parent get passed on - they may line up in any combination.
An example of what I am talking about - most people are aware that a mule is a cross between a horse and a donkey. It is generally accepted that mules are sterile, but there have been several female mules that have given birth. In one case, the father of the mule's offspring was a donkey, and out of curiosity, the owner had the foal DNA tested. The results showed that the mule's foal was 100% donkey. It had (obviously) gotten donkey chromosomes from its father, but though it could have gotten horse chromosomes from its mother (which was half horse), in this particular case, it didn't. Some mule's foals have tested as part horse, part donkey; but the percentage doesn't necessarily work out to 75/25.
With the complex crosses you are talking about, it's quite possible that some resulting offspring will have no genetic material at all from some of their "ancestral" breeds, if the purebred birds are far enough back in the cross.
