It all depends on how much Alk and Calcium your reef is consuming. Many folks use a CA reactor when tuth to be told, they don't really need it. Basically you will have to determine your consumption. Also, what needs to be understood is that a calcium reactor is more about alkalinity than calcium.
My recommendation would be:
Use this calculator: Dosing calculator
Step 1 the Magnesium part.
Magnesium is also an important element in skeletal growth as well as in balancing properly CA and ALk. I try to maintain my levels at around 1300 PPM.
Also Use the calculator above in order to add the proper amount of MG supplement, to reach your target, and until you have regular target levels readings. Bear in mind that many salt brands are rather depleted in MG levels. You may thus need to dose regularly until you can maintain your targeted MG levels.
Step 2 The Alkalinity part. Do not dose calcium yet.
Set a target for your desired alkalinity, 8, 9, 10, or 11 DKH. Dose accordingly until you reach that level. Once you reach your target, stop dosing Alk. Then do daily testings, for example every evening, to determine your daily Alk consumption. Once you determine your daily consumption, dose again daily , but only the amount needed to maintain the target level.
Step 3 The Calcium part.
Do this only after having completed Step 2.
Test your current calcium level. Once done, use the calculator above to determine how much CA you need to supplement in order to obtain a balanced level of CA to ALK (ex for my 100 gal systems, alk = 11 then CA = 439). Once you reach your target, stop dosing CA. Then do daily testings, for example every evening, to determine your daily CA consumption. Once you determine your daily consumption, dose again daily , but only the amount needed to maintain the target level.
my $0.02