np dude.
Just checked up on the formula there, he claims that the formula has a balance by accounting an excess 12% by weight Mg/Na.
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After adding 594 grams of baking soda (1 gallon of Recipe #1), we will have added 163 grams of sodium. In natural seawater, magnesium is present at about 12.0% of the sodium concentration (by weight). In order to match the magnesium additions to the sodium additions to leave them in a natural ratio, we need to add 12% of 163 grams, or 19.5 grams, of magnesium for every gallon of the two-part additive that we add. | |  | |  | |
As I mentioned earlier, he also speaks of how potassium in the dowflake contributes to the residual ions.