I have found that using an app is helpful in determining whether I am consuming all of the necessary nutrients in a day. I use cronometer.com and think it's terrific (easier to use on the computer vs the phone app). It's really eye-opening. I've been inputting my daily nutrition for years but when I miss a day or two, I still have a feel for what I need based on past input. You'd be surprised at what nutrients keep coming up short. I use the daily information to supplement accordingly, taking a multi and calcium, etc only when I've fallen short.

There are nutrients that compete for absorption like calcium and magnesium and calcium and iron but I have to think that when we consume foods in their natural state, they contain a multitude of nutrients that "compete" and that our bodies likely know how to absorb and utilize those nutrients provided our digestion is good. That's just conjecture as there isn't really good data on this and nutrition information keeps evolving and changing. As an example, we had tofu for dinner last night and my husband said "Wasn't tofu good and then bad? Is it good again?". As far as competing nutrients or constituents, there are some extreme examples like spinach which is high in iron and calcium (competing nutrients) and also high in oxalates which inhibit the absorption of calcium so best not to rely on spinach as a good calcium source but rather an occasional food. (by the way, that cottony feel in your mouth when you eat spinach is the oxalates)

Calcium is utilized in the body throughout the day and there's a blood/bone exchange that happens regularly. Your body will always seek to balance your blood levels and so if necessary, will pull calcium from your bones. That is why when you have your blood calcium level checked, it is generally in the normal range. If it's not, it's definitely something to explore further. Calcium is best absorbed in smaller doses throughout the day. They say at a maximum of 500 mg at a time but I think lower doses, around 200-350 mg at a time are best particularly since it is believed that excess supplemented calcium can potentially lead to calcifications of the arteries. And yes, citrate is much better absorbed than carbonate and can be consumed without food unlike carbonate which requires food intake.