I have free choice oyster shell in the coop but I am still having some shell weakness issues with 1 or 2 out of the 10. Some mornings there is a broken egg on the roost board or below it. They get their light turned on by 6:30 am and it is on til 6:30 at night. They get a small amount of scratch most days but mostly all they eat is their layer food. I would like to add calcium to their feed. How much per pound would be right? What can I do about the night laying?