tell the truth, before I found a store that sold purina gbs 30%, I was feeding them dumors show feed for chickens, its was a high protein ~26%, then I found frm gbs 28% and started mixing they did well and their feathers looked great. now I cant get the showbird feed. if I could I would mix it 50/50 with the gbs for the older birds.
I have also tried mixing in catfish fingerling grower crumbles 38% protein with a total different protein source (fish) once and have been thinking about that again if its cost effective.
so , all that said, its like the quick and easy is the purina gbs 30%, add in some crushed oyster shell, bird seed, cracked corn, anything else you want, if you want.
simple math tells us to give them just as much or more calories per day than they need, with a proper complement of amino acids, vitamins and minerals.
throw in the egg laying and they can certainly turn over some nutrients, and then the more nutrient dense the feed is the less, they have to eat and produce waste?
I agree the egg laying is a different need than the meat bird growth stage, gathering some data from some egg laying chicken threads may help us here?