Very informative!
Vitamin D is also part of this equation, as phosphorus and calcium both need Vitamin D to absorb them.
This article could be improved by providing citations to articles to back it up or providing further reading to anyone wanting to learn more.
Here is a great article by Grubbly Farms that explains the chicken's need for all three: calcium, phosphorus, and Vitamin D.
In a perfect world, folks would feed their chickens a good all-flock (supplementing oyster shell for the layers) or layer feed that contains enough vitamins, minerals, etc. for chickens and laying hens, and then store it properly in a sealed container, and ensure never feeding spoiled, moldy, or extremely expired feed. Unfortunately, it's not a perfect world, so good if folks understood the correlation between calcium, phosphorus, and D, especially if they're having issues with their chickens as you described.