Some bread is a good deal more nutritious than other bread.
It would probably be safe to count it as a "treat" anyway. Some animal nutritionists say "not more than" 15% of their diet for any thing other than their rations.
I got an idea about what might be a reasonable amount of treats (scratch, scraps, garden veggies) by weighing the hens' feeder as it was filled each day. With your 5 chickens, you could say that they should eat about 1 1/2 pounds of their commercial feed, daily. Give them their treats, then see if they are "cleaning their plates" each day.
If they are getting so much bread that the 5 birds are only eating a little of their feed, you are probably overdoing the bread.
What I found was that I could give my hens far more treats during the winter time than I had been. During cold weather, they are really chowing down and it is just to get enuf calories to keep them warm. Of course, we are moving into the time of year when they may need cooling down rather than warming.
Steve