Thanks for the complete answers. That definitely helps. This is going to be rather long, but hopefully you will learn some useful info that will benefit your flock.
I presume "Crushed oyster shell and crushed granite grit are available at all times." means that it is on-the-side and not mixed in with their other feed, right? This is actually important as different birds need different levels of calcium and on-the-side lets them regulate their own calcium input. Likewise, chickens will swallow grit as needed to help with their digestion. Since chickens don't have teeth, they use the grit in their gizzards to grind up their food.
There are two major schools of thought on how to best feed chickens using commercial feed. One is the "use layer feed because I have layers" group, which I used to be in. The second is the "use flock feed + calcium because I have a mixed flock" group, which I switched to a couple of years ago. By the way, chick feed is basically the same as flock feed except that it is crumbles.
Nutrition standards for commercial feeds have been set to support commercial meat/egg operations, not backyard chicken owners. That is what causes the two different feed strategies I mentioned above. Commercial chicken outfits want the cheapest feeds that will allow for quick chick and/or meat chicken growth (2-4 months depending on breed). Those feeds have higher protein (18-22%) and minimal calcium (~1%). Commercial outfits also want the cheapest feeds that support egg laying for two years as production starts to decline by their third year. Those older layers are then dispatched. The layer feeds have minimum protein (16%) and higher calcium (~4%).
Now understand that commercial operations don't care about the long-term health of their birds as they don't keep them very long. They want them to grow to a certain size quickly and/or produce a certain number of eggs for the lowest price. They don't care about long-term health of the birds or the quality of the nutrients in the eggs. Those are things that are generally more important to backyard chicken owners.
So, lesson #1 is that 16% is a minimum, not necessarily ideal protein level for adult birds. You already understood that at least partly because you offer them higher protein to help them regrow their feathers during molting when calcium isn't as important, but more protein is.
The lesson #2a is that 1% calcium is perfectly fine for growing chicks and maintaining adult chickens. The 4% is only needed to support hens massive needs to create so many eggs. Lesson #2b is that 4% calcium is actually bad for any chickens not actively laying. Too much calcium can actually create painful joint problems over time.
For commercial operations, this is really simple. Feed chick starter/flock feed until you want to harvest the chicken or they start laying. If they are layers, then switch them to layer feed until you want to cull them. Easy peasy. Chickens are kept in a very artificial environment with controlled temperatures and lighting, so they don't even have to worry about them stopping laying in winter or getting old enough to deal with too much molting.
For backyard chicken owners, things can be considerably more complicated. You may have different ages in the flock that have different protein/calcium needs. You may have one or more cockerels who never want/need the extra calcium. You may have aging hens that are laying less often, so they don't need as much calcium as a young layer. Your layers go through different phases where they need more calcium during active laying periods and more protein when molting. Even if they don't have a serious molt, they will usually stop laying in winter when days are shorter, so they don't need as much calcium.
One way for us to handle this is to try an have a uniform layer flock. We can start with a high protein mix when young, switch them all to layer at the same time and then switch them all back to a high protein mix in winter. This isn't perfect, but works fairly well. It is where I started and seems to be what you are doing. Once I started having a mixed age flock, with some some non-layers (young), some active layers and some less active (older), I realized I had a problem using that layer feed system.
I did some research here on BYC and discovered that the "flock feed + calcium" system actually is *much* better for mixed flocks. You offer a higher protein feed (18%+) which gives your hens a better reserve for when they start molting and for their general health. That same feed has lower calcium, so all of your birds can eat it safely. To help your layers, you offer calcium "on-the-side" and they will have a natural drive to consume it to support their needs, much like animals naturally desire salt to support their own bodies needs. This setup works with mixed ages, mixed sexes, during different seasons, etc. The only problem is for chicks who need crumbles rather than the full-sized pellets, but that is fairly easy to handle when needed. This is the "easy peasy" solution for backyard chicken owners.
If you use that latter system, then the feed will handle everyone's varying calcium needs. The only time you use a calcium + D3 tablet is to handle rare egg problems.
Since you have a mixed-age flock, my bottom line recommendation is to not worry about tablets, but to switch your flock to flock feed + calcium and grit on the side.
Sorry for the long post, but I think understanding the why behind my recommendation will benefit your flock.
