Just take a sample of water down to the store when you go to pick up the corals and have them test your levels. Keep in mind once you start messing with calcium, you will start a balancing game. Alk, calc, and pH and those all have a pretty complicated balancing relation. Add too much of one and the other will precipitate out and cause a salt water snow storm effect. Just make sure to read up on it before you start tweaking levels beyond water changes.
I can't imagine your calcium being off far. I would be more concerned with your alkalinity (measure of carbonates - bigger component than most realize, calcium carbonate is only 1 calcium to two carbonate if my chem serves me well). I normally keep mine a bit high. Alk tests are very easy, cheap, and come with most pH tests, I would recommend one.
I normally test pH and alk. If they are off, my calcium is normally a bit low too. I stick with my 5% water changes every couple weeks and I also have a jug setup with a couple drip valves for kalkwasser when needed (concentrated calcium something or other).
In a nutshell, just have them test, but if you do your water changes at all, you should be more than fine =). As you start adding more and more and if your levels become affected, you'll have to do some reading.
Good luck =). Enjoy your zoos... I'm a huge fan of my red shrooms in my tank.
