I would suggest feeding the chickens layer feed instead of all flock feed, as that would help with the calcium issue and create stronger eggshells. I read that your feed only has 16% protein. The correct amount of protein for chickens to be fed is 18%, and unfortunately almost all bags of feed now contain 16% rather than the proper percentage because it is easier for the company to produce it. I feed my chickens cat food on the side as well—the bag I give my chickens contains 30% protein, which is a lot more than they need, but make sure you are only feeding them a small bit and they will be fine. I have done this with my chickens and they all are in great feather quality, health and produce good eggs.