I have been feeding it for about 6 months to increase the quality of feathers, I also ferment my feed. Usually add about 3 cups ground to my 5 gallon feed bucket, I like protein around 18% for maintenance. I am bumping it up to 20% for molting. This is based on the research I have done for feeding feathered feet chickens (Cochin & brahma) most feed around 18% protein. It is fish meal and soy meal based for the protein and I know some worry about egg flavor, I personally can't notice any difference.
My flock is still young not 2 yet, learning from those with many years experience and putting it into practice. My first year was just the basic leave out dry feed and water. This year I have been researching ways to make it better for me and my flock.
If you keep the chickens past two years old, the feed formula you are using now may be too much for them.
Purina is working on a chicken feed for Senior chickens because of this. For production, chickens are not kept much past two years old. Back yard chickens are often pets so are kept much longer.
Good job on the research!