Your magnesium is where it should be, somewhere around 1250 to 1400 is fine.
Calcium at 370 is nothing to panic about, but if you want to maintain it at 400, adding some calcium is the right thing to do.
Something funny about your alkalinity though. Are you talking Mg/L or DKH? Either case the numbers you mention seem unlikely especially as you had a high pH. If you know any other reefers you should ask him to test a sample of your water to double check the accuracy of your kit, or if you don't know anyone yet, take a sample of your water to your local marine fish shop and ask them to test the alkalinity for you. Once you have established the correct alkalinity then post it here and we can advise what to do next.
BTW, if you don't yet have any livestock in the tank, it will not matter much what your calcium and alkalinity are at this stage.