I offer oyster shell free-choice out in the run. It disappears in a slow, steady stream. The hens know when they need it. In addition to eggshell they get layer feed (which has higher levels of calcium than starter feed), sunshine and plenty of table scraps.
Same here, but mine is in the coop. I nailed a sour cream container to the wall - it's perfect for holding oyster shell. They will take what they need - if you mix it in their feed, they may not be getting enough.