If you use Duramycin for anything, use 4 tsp per gallon. Don't try to figure out the dose on the pack. 4 tsp per gallon is the only dose that will do any good. It will not hurt them to use that much. You can use the same dosage for chicks. And btw, wait three weeks after they are done with...
OK, one more thing. If you have a high mortality rate, chances are it is ILT. This is bad, but...there is a vaccine you can get to give them, even the sick ones, which will help a whole lot, and the vaccinated birds will NOT be carriers. I believe it is a dead vaccine, but again...
It could be any of the things already mentioned. If one dies, you definitely need to get a necropsy done within twenty four hours. If one is on it's last legs I would go ahead and take it for a necropsy and just have them euthanize it. Painful for you, but it may save the rest of the flock...