Pay very close attention to the poultry feed you are feeding. Much of it now is certified vegetarian, meaning you are not feeding your chickens chicken meal. That eliminates any possibility of the 'mad cow' issues.
There are two main sources of protein in your chicken feed. The first is grain, which contains varying percentages of protein (corn is the lowest around 9%). The grain used will be whatever is cheapest at the time of milling. It can be wheat, barley, oats, corn, etc.
The second source is seed meal. This is what's leftover after seeds have been pressed for their oil. The bits of seed average around 40% protein. So as you can see, it's simply mass to multiply the protein percentage by weight to get your desired protein percentage.
The other source not mentioned is fish meal. It's great stuff. Chickens are omnivores, they do need some meat in their diet. I wouldn't hesitate to use fish meal; but I use it as a top dressing since their feed is a complete ration.
Other easy sources of protein for your birds:
1) If you are using pasture rotation, rototill a strip of ground in their 'pen' each week. They will feast on bugs, worms, etc, as well as fertilizing the ground.
2) Legumes. Take your over-ripe peas or beans and throw the whole stalks and pods into their pen. They will pick the peas/beans from the shells.
3) Grow a forage crop for your birds. I let the chickens loose in my field containing oats after I had let the pigs at it. They did a great job cleaning up.
4) Roadkill. Place it in a bucket and drill 1/8" holes in the bottom and a few inches up the side. As the maggots are formed, they fall on the ground where your birds will devour them. They are extremely hig in protein; and you can just harvest protein from thin air! Amazing. Not that I'd ever do it.