the best way to figure this out is to do redundant testing. test it at the same time every week or even everyday, and see how much the calcium drops during the week or day in heavily stocked systems. Then you can figure out your dosage.
Never, ever, add anything to your system you aren't testing for. Taking calcium or alkalinity too high can cause a "snowstorm" where the calcium precipitates out. It takes weeks to recover from this as the system will be prone to doing it again if it happens once. You can't just dose the calc right back up because the snowstorm will happen at a much lower calcium level. (the snowstorm is pretty, but I don't suggest it, as many things die as a result of the pH swing.)
I don't mean this to be harsh, but I really want to drive home a point here. The downside is tremendous, and the upside may not even be noticable, as it is quite posssible that you don't even need it yet. Usually a tank will need Alkalinity supplementation weeks or even months before it needs calcium supplements.
Start doing super consistent testing. For my system I do basic testing 2x a week, more if there is a problem I am trying to adjust. Basic testing for me includes nitrate, phosphate, alk, pH, salinity, calcium. Honestly, its boring as heck because the numbers just don't change much usually. But its a must, especially if you are considering dosing. I do it to keep an eye on that pesky Alk mostly. The other test I probably should be doing is Magnesium. I will start in the next month or so as my stony corals get bigger and more demanding.
Hope that helped. oh yeah, the Kent products seem ok, not my favorite, but I know other reefers who swear by it.
