I put mine on layer feed when I get my first egg. With the adult birds I have now that was at 19 weeks, with my juveniles it was 18 weeks. When my hens are molting they don't visit the oyster shell pile. It's as simple as that.
Every feed bag I have ever read says to start layer feed at 18 to 20 weeks. There's nothing on the bag about switching them back to a grower or other food if they are molting, etc. Why do you think the feed manufacturers would recommend it if it wasn't the sensible thing to do? It's not so they can make more money off of the layer feed. Here, layer feed is the least expensive of all the chicken feed available. It's not so your chickens will die off from getting the layer; then they wouldn't be selling you any feed at all.
Layer feed has a low percentage of calcium as it is (again, going by what's available here) - 3 to 4%. Nearly all living creatures need some calcium in their diet, regardless of whether they lay eggs or not. In humans at least calcium is necessary for contracting muscles, forming and strengthening bones, conducting nerve impulses throughout the body, clotting blood, maintaining a normal heartbeat, as well as other important processes.
So, I keep it simple. I feed a starter/grower until 8 weeks, then a grower, then layer feed when they start to lay eggs. I have two very healthy flocks; including some CX rescues that were to be culled at 3 weeks old for not growing fast enough.
Before you hit me with all kinds of studies concerning commercial flock practices, let me tell you that I'm not interested. What I am interested in is keeping my flocks in a way that is as far removed from commercial practices as possible. My birds free range all day, have a safe, dry coop at night and layer feed available in their feeders whenever they want it.