I agree with the others about putting the oyster shell separately.
I do mix my oyster shell in, since my feed has zero calcium added (homemixed) and if I don't the eggshells DO get too thin. They are perfect with the shells mixed in. They do leave them at the bottom of the feeder and don't...
Personally (I am not a chicken nutrition expert) I don't mind it if the chickens eat a lot of BOSS. You do need to be concerned about fatty liver disease from too many seeds I have read, but I have found that (because my feed mix is very heavy on seeds) my chickens always get tired of seeds and...