This is taken from Randy Homes Farleys recipies
Recipe #2, Part 2: The Alkalinity Part
Dissolve 297 grams of baking soda (about 1 1/8 cups) in enough water to make 1 gallon total. This dissolution may require a fair amount of mixing. Warming it speeds dissolution. This solution will contain about 950 meq/L of alkalinity (2660 dKH). As mentioned earlier, Arm & Hammer is a fine brand of baking soda to use in these recipes. Be sure to NOT use baking powder. Baking powder is a different material that often has phosphate as a main ingredient.
Found here:
http://reefkeeping.com/issues/2006-02/rhf/index.php#11