For feeding chicks up to laying hens: There isn't just one answer, it depends on what brand of feed you bought and how they formulated it. Read the recommendations on your bag if it has them.
Chick starter should be feed up to 14 to 22 weeks, grower from 14 to 22 weeks, and layer after that. Layer can be started as early as 16 weeks. Just so long as you are feeding layer when or a little earlier than the hens start laying eggs. Chickens start laying from 16 to 26 weeks depending on the breed. You should look your breed up to find out when they should start laying.
Yes the numbers don't work out and are very confusing for beginners. If you only have a few birds it is recommended that you feed starter until the bag is empty. Then feed grower until the bag is empty then start them on layer anytime after 16 weeks.
If you have only a couple birds and it looks like the feed will last long after the 20 weeks you can mix the feeds 10% at a time until the feed is gone. Or just add a calcium source to the feed until the feed is used up.
Or you can buy Starter/Grower mix and feed it until the layer feed is needed. That is what i got for my very small flock. I expect the bag to last until i can start the layer feed.
Something else to consider when thinking of changing feeds. Chickens need 12 hours of light a day to lay. We start getting 12 hours of sunlight the third week of March. And during the third week of September the light shortens to less than 12 hours of sunlight a day. So you need to adjust your feeding schedule to work with the light, or add artificial lighting to extend the season.
Don't worry about feeding layer to roosters. The extra calcium wont hurt the birds. The extra calcium isn't adsorbed, going through the bird unused. When you see chicken droppings that has a white covering on the dropping that's the unused calcium. I would worry more if my hens didn't show the white covering once is a while. I use it as a indicator that more shells are needed.
If you use eggshells as a calcium substitute they must be crushed so fine that the chickens cant tell that they were once eggshells. If they can tell they are eggshells some chickens will start eating their own eggs if they are short on calcium.