[quote author=robert4025 link=topic=4311.msg52238#msg52238 date=1218670897]In anycase, occasional testing of your Mg level is highly recommended, at least until you have a baseline of how much your system is consuming it.[/quote]
Mg is consumed at a specific rate relative to alk/Ca in calcification. If you know your alk consumption, you can calculate your Mg dosing rate(as well as your Ca rate).
Here's a good article on Mg:
http://www.advancedaquarist.com/issues/oct2003/chem.htm