If you put oyster shell out in a separate feeder, available all the time, they will take what they need when they need it. Like many others I use flock raiser rather than layer feed, have been doing so for years, the extra protein helps, and it also helps ensure that those birds that don't need the extra calcium (roo's, young birds, birds in molt, birds off laying for the season) will not get more calcium than they need. It is always a good idea to provide grit the same way, in a separate feeder, that way if for some reason they don't get it adequate from the environment, it's available to them.