I too use Purina starter grower followed by Purina Flock Raiser when they go in with the big girls. I have oyster shell available free choice. Other than the occassional wonky first egg, all of my hens have good hard shells. I actually dropped two eggs on the ground outside last week and they didn't even crack! I like the flock raiser because I seem to always have all different ages of chickens and its easier than trying to feed them different feed.
Worked out great when someone gave me a duck last week as the feed is appropriate for it as well.