You must analyze all the waste products to gain the values of what has been absorbed. Set a baseline first, give supplement, analyze all waste for calcium, include blood levels for comparison data, and keep this up until the the baseline is once again present.
Calcium absorption is influenced by many factors, such as the composition of food eaten by people. Thus, measure the percentage of calcium absorptions is difficult.
Calcium supplements are well absorbed even by the tiny preterm baby the amount of the net retention depends of the Ca/P rario and the used calcium salt. See classical paper: Moya M, Domenech D. Role of Calcium-Phosphate ratio of milk formulae...Pediatr. Res1982; 16: 675-81
The golden method to measure the percentage of calcium absorptions is to meaure calcium intake and fecal calcium excretion associated or not with calcium stable isotope intake. However, one can have a rapid idea about calcium absorption (comparative) by measuring 24 hours urinary calcium after ca intake.