The general idea is that you want the chickens to have a balanced diet. The proportions of protein, fat, fiber, salt, vitamins, minerals, and such need to be somewhat close to what they should have. You don’t have to be real precise. I’ve never seen a chicken see a grasshopper and say “No thanks. I’ve had enough protein for the day”. The prepared chicken feed is a balanced diet.
That’s where the 10% of their daily diet comes in, not that something is bad for them, just that it upsets a balanced diet if they eat a whole lot of the same thing. Too much of a good thing is not necessarily a good thing. Think in terms of moderation. Also vary what you are feeding them a bit. Maybe some kale, then some corn, then some meal worms, then some cabbage; just not a constant diet of the same thing.
Personally I like for the chickens to get things other than the prepared chicken feed. Mine get a real variety from the garden during growing season and a lot of different kitchen scraps now. I keep a special bowl on the kitchen counter for anything the chickens can eat and feed that to them once a day. That way they get a real variety daily.
When chickens can forage they do eat a lot of stuff, but the variety of that depends on the quality of forage. If you have enough different grasses and weeds, grass and weed seeds, and plenty of creepy crawlies for them to catch, they can live off of that. That really helps your feed bill. If the best you can do is a manicured back yard with just one variety of grass that is never allowed to go to seed, they will still get a lot of nutrition and have a lot of fun in that. That’s better than a lot of chickens get.
