As matter of fact I am not a huge believer that even laying hens need a calcium content between 3.5 and 4.5 percent anyway. I used to always mix layer and flock feed at 50/50 and have always had very good luck with egg shell hardness and healthy birds. People think that hens were fed just what ever grains were spilled and free ranged for bugs and such was "way back in the old days" LOL. It has not been that long ago. What ever natural calcium they could get was way less than the 3-4% that is in layer feed. The soft shell egg was very rare even on this diet.
I believe that the feed companies were responding to the commercial market to strengthen the eggs for the rigors of transport and handling when they started fortifying with so much calcium. I think this became SOP and was accepted as normal for even the back yard folks like us.
If I ever get tired of the long drive for a blue seal dealer, I will probably go with purina non medicated chick starter for all my breeders and laying flock. 18% protein, a little over 1% calcium, decent amount of methanone, marigold extract, and vitamin base is about the same as blue seal breeder. This also will be a whole lot better for the roosters that especially dont need all that calcium either.