I think the standard advice would be to add Alk supplement only until you achieved "balanced Ca/Alk"...What you consider to be balanced is maybe a bit subjective... Let's say it is around 400-440 Ca...and about 8-9DKH.
Once you have that, you should maintain it with a balanced additive (like A/B, Kalk, Ca Reactor). Then use this balanced additive to maintain these levels. A dropping Alk level serves largely as an indicator that you are not replacing BOTH...even though the Ca level appears to be holding.
To quote Holmes-Farley..."The "mechanism" arises in the simple fact that alkalinity rises and falls much faster than does calcium because seawater has a much bigger reservoir of calcium than it does alkalinity."
A good recommended read:
http://reefkeeping.com/issues/2004-12/rhf/index.php
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400 gal reef. Established April, 2007. 3 Sequence Dart, RM12-4 skimmer, 2 x OM4Ways, Yellow Tang, Maroon Clown (pair), Blonde Naso Tang, Vlamingi Tang, Foxface Rabbit, Unicorn Tang, 2 Pakistani Butterflies and a few coral gobies
My Tank:
http://www.canreef.com/vbulletin/showthread.php?t=28436