I have never really considered putting lots of work into feeding my chickens, they have 20 acres to forage on during the day. And I feed them a bit of Purina feed in the evenings. Have been for years 

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agree with this. Many factors21hens-incharge brings up a really good point.
Breed differences also factor into nutritional needs. Some breeds do much better than others on foraging. Some breeds simply CAN forage better than others.
And how rich your ground is for forage makes a huge difference too. Flock size vs. ground size and forage availability factor in.
There won't be any one formula that says this many birds with this forage equals this.
Each keeper will have to carefully factor in their specific flock needs.
One head up warning, chicks started on bagged chick start often have trouble transitioning to straight forage. They can get spoiled with the easy to access and digest "canned" food.
I have free access to leavings from a produce store...only my spoiled girls, having been raised on bagged feed, turned their beaks up at it. Others have had similar issues.
I don't have enough free forage for my birds to not give bagged feed, so it became a losing battle for me.
Just something to think about too.
LofMc
Why not just get your own custom feed mix from a feed mill? Lot of recipes available for those...
I mean, when people say they want to feed their chickens less grain... Do you think chickens eat less grain foraging when given the option? I dunno about ya'll but my chickens loooove them some grass seeds. Which is like grain before we made it big. Thistle seeds are a big hit too. Grain is just plant seeds. Flour is just ground plant seeds. Some seeds are more nutritional than others. Corn has a lot of sugar and very little incomplete proteins. Oats are better. Peas aren't a grain, but are packed with proteins. These are things chickens would eat in the wild when given the option to. They're designed to digest seeds just like any other bird.
I get a desire to avoid low quality grain (like corn) or a desire to avoid feeding only grains. But chickens eat cereal grains and love it. Spent brewers grains are an awesome chicken feed source. You could probably put together your own custom mixed feed by buying a few bags of this and that and mixing them in a big drum and feeding it out from there and have perfectly healthy chickens, no cooked or ground feed and healthier grains.