If you're worried about keeping stony corals, your levels are fine IMO. I think the optimal levels are something like Ca 440ppm, ph 8.4-8.45 and alk 3.5 meq/L. As long as these 3 things are in the right neighborhood, everything will be fine. If on the other hand you have a Ca level of 450 but a low PH or Alk then your corals won't grow. Another much easier way of knowing that you have the correct Ca/Alk/pH going on in your tank is if you have coralline algae growing. I'd check out the FAQ at the top of this board for more info on Ca and Alk: http://reefs.org/phpBB2/viewtopic.php?t=7679
