1/2 gal package is dry powder ca and alk, makes multiple gallons of liquid Ca and Alk
dosing depends on size of your tank and consumption. Before you start you should have good test kits for Ca, Alk and Mg. You will need to maintain mg above 1200 to maintain alk and ca levels.
LPS and SPS will use Ca and alk to build their skeletons. The amount you need to dose will depend on how much LPS/SPS you have. I think your tank is 120gal? With a small amount of sps/lps the 1/2 gal package will probably last 3-6 months