You said you are using instant ocean. I just tested some and got a Magnesium of over 1000 (1250 to be exact) which is hunky dorey for a good calcium level. So... I revert to my original recommendation, use more additive. A good water change may be in order to start with prior to adding calcium if you have a decent bio load of calcium using animals just to ensure your Mg is sufficient.
I recommend a 2 part product like E.S.V. Bionic or Ocean's Blend or another one of the many products out there. The bennefit of these is they tend to keep your ionic balance in check while raising calcium. Turbo calcium WILL raise calcium very well. You will end up with more Cl- ions floating around your tank than Natural Sea Water has though. I really am not aware of having any serious issues with this ionic imbalance, but I have read enough from others with more experience than I that recommend keeping it in check, and this makes me take heed.
I was in your shoes with my first reef tank. I was leary about adding too much. I eventually got over it (with reason) and added more. If you can add and test your results, IMO, you have nothing to fear since you can measure the increases to prevent overshoot. Dose, wait for proper mixing, test, get an idea of how much Ca increases with the dose, rinse and repeat until you are where you want to be.
