The only feed we get reliably around here is Purina. So I keep Purina Turkey Starter around (26% protein, salt and sodium is the same as chicken feed) and mix it in different proportions with Layena, depending on how the flock is looking and laying. Layena is 16% protein here. And about once every two months I buy a 7.5# bag of kitten chow (usually around 40% protein), cheapest I can find that has an animal product as the first ingredient, and for 20 birds I give about a fistful maybe twice per week spread out for them to scratch around in the bedding or outside. I limit the use of corn scratch to days when they have been in the coop a lot due to bad weather. I use corn more for entertainment than for food treat. One 50# bag will last about 6 months around here, kept clean and dry of course. Beyond that they get a small amount of kitchen leftovers and/or veggie scraps from the kitchen or grocery castoffs. And small is a 1-gallon container for 20 birds maybe once per week. Also if they are coop-bound due to weather I will soak a pan of alfalfa cubes in water for them to peck at. Lastly, plenty of fresh clean water.
Mostly I try to keep the micromanaging to a dull roar. If they are looking poorly, going thru moult, stressed...I increase their nutritional rations. Really bad, egg production almost to 0...add in some kitten food. If the weather is great and bugs abound...just Layena.