Hi Fresh,
I never had a Calcium problem when I had only a few LPS and softies, but as mentioned your best guage is a test kit. That's with bi-monthly water changes of between 5-10% each.
I now have a small variety of SPS and LPS (mostly in the form of SPS frags and 1 clam). I find that I supplement liquid calcium a couple of times each week and keep it at around 440-460 (60 gal tank). I do not run a Ca reactor, nor do I really believe that they are necessary for a softy tank, although I cannot wait until I have the budget for one now that I am adding more SPS
Good luck,
Rob