It is just about impossible to anticipate how much, say, calcium we will absorb from a meal or a pill. It depends upon a number of variables which themselves can be difficult to measure, including but not limited to: our calcium status, how much calcium is in the meal or pill, what the calcium is bound to, how acidic the internal environment is, what our vitamin D status is, how much of the calcium binding protein is in place, how much of and what type of fiber is present, the presence of other divalent cations like zinc or magnesium, the condition of the intestinal brush border.
The amount of calcium (or any nutrient, I'm using calcium as an example) consumed and the amount absorbed is not a linear relationship. That is, eating 100 mg of calcium does not mean 100 mg gets absorbed. One thing that is known - the more calcium we eat, the less we absorb; and the less we eat, the more we absorb (relatively, not absolutely).
This applies to the macronutrients - carbohydrate, fat, and protein - as well as the micronutrients. Eating 20 grams of carbohydrate does not mean 20 grams gets absorbed. Some of that carbohydrate may be in the form of a nondigestible fiber or resistant starch, for example.
Compounding the uncertainty, nutrient content varies within food - the same type of food. In this study:
Calcium and Magnesium Concentration of Inbred and Hybrid Broccoli Heads, Journal of the American Society for Horticultural Science, 2000
Farnham et al. found significant variability among broccoli varieties for calcium and magnesium. Broccoli purchased at random across the US varied by as much as 2-fold for these elements.
Here's a study that shows almost a three-fold difference in calcium content among several varieties of kale and collards. It also shows significant year-to-year variability for the same variety:
Variability In Elemental Accumulations Among Leafy Brassica Oleracea Cultivars And Selections, Journal of Plant Nutrition, 2005
And yet, data bases and nutrient tables give calcium content in a cup of cooked kale to 2 decimal places!
Therein lies the challenge of structuring a diet with a reductionist mindset - armed with an RDA or DRI (which are padded with thick margins of safety) in one hand and a food nutrient database in the other. It can easily lead to supplementing "to make up the difference" or "as an insurance policy," when supplementing can ironically result in nutrient deficiencies (e.g. when nutrients compete for absorption) and undesirable side effects (e.g. overdosing on antioxidants (like vitamin C) can block important metabolic functions).
What might be more effective is to look at whole diets of large groups of people and correlate dietary patterns with health and disease.