Good information, a bit more complicated than I think it needs to be. I have simply put out a bowl each of oyster shell (crushed and sold for poultry) and grit since my first chicks reached 16 weeks of age. Many moons ago. I feed a layer ration (16% protein usually and feather fixer during molt) and let the girls eat the supplements as they want, with the oyster shells placed high enough that chicks can't reach it. Once I get enough eggshells I crush those and pour some into the dish. I've never had any problems with the chickens or their eggs. My hens have free access to their run, maybe 12'x40', so they can get some "grit" there too.
