Feed oyster shell free choice. It doesn't matter if you are feeding layer ration always offer oyster shells. My egg shells are so hard, its almost hard to crack them when I'm cooking.
Hens with thin eggs sometimes don't like to eat enough oyster shells. You can mix a small amount of oyster shells with can cat food and feed it once a week to insure the hen is getting enough calcium. Just don't over due it, hens only need a small amount.