Keep a log of test results and what and how much you add to correct imbalances.
Here are some articles on reef chemistry:
Solving Calcium and Alkalinity Problems:
http://www.advancedaquarist.com/issues/nov2002/chem.htm
Magnesium in Reef Aquaria:
http://www.advancedaquarist.com/issues/oct2003/chem.htm
Tank Volume Calculator:
http://home.comcast.net/~jdieck1/volcalc.html
The Reef Chemistry Calculator:
http://home.comcast.net/~jdieck1/chem_calc3.html