I have 6 Barred Rock and 6 Black Sexlink hens that are 18 weeks old. I just put the last 35# of the starter/grower in the coop. This should last them about 2 weeks. I am a bit confused as to when I should switch them to layer. I understand that the extra calcium if they are not laying yet is harmful, but I have read that they might lay soft shelled eggs if they don't have the calcium.......So, when do I switch??? Thanks