I would not worry about it too much either. Once you have coral in it, I would paid more attention to Ca and Alkalinity level. Bulk Reef supplies sell the 2 parts additive that is really good and it is what I use to keep my Mag, Ca and alkalinity stable. Actually it is 3 parts because they have pre-portioned Ca, Alkalinity and magnesium. You can mixed them in three 1 gallon containers and add the same amount of these additive to tank on regular interval to keep the level where you want it. By far I find this the easiest to use.

http://www.bulkreefsupply.com/aquari...portioned.html