Calcium and phosphorus digestibility in poultry diets

Excellent paper Stuart.
Right now the only thing that I am sure of is that almost 99% of Ca is stored in the skeleton as hydroxyapatite (Proszkowiec-Weglarz and Angel, 2013) and that hydroxyapatite has a Ca:P relationship of 2Ca:1P.
If today, when formulating by total calcium, we tend to exceed the calcium requirement of the formulas, we will do so to a greater extent when formulating by digestible calcium.
Am I right?

Dear Dr. Wilkinson. Very interesting material. The variability of limestone in Australia is quite similar to what we see in Brazil, including micro-minerals like iron. One major problem I see with calcium digestibility is how, different form other nutrients, including phosphorus, digestibility is highly influenced by the amount of calcium in a complete feed, as you have mentioned. This is mostly caused by a tight regulation of absorption, different from P where what is digested is absorbed and excess excreted via urine. In calcium, the main route of excretion is excreta. This makes me think that determining calcium digestibility of individual ingredients is not enough because additivity would be a problem. The final digestibility of the feed is dependent partially from the ingredients but also from the total amount of calcium in the diet. Would like to hear your thoughts on these considerations.