In a gallon of warm water, you could get about 1.5 grams of dolomite to dissolve... (well, about 1.4 grams of MgCO3 and less than 30 milligrams of calcium carbonate, or less than 700 milligrams of Mg, and less than 15 milligrams of Calcium), but most of the calcium isn't going to be soluble.
You'd be way better off getting a nice chelated fert with micronutes in it, or adding dolomite to soil before you plant. Epsom salts for Mg wouldn't affect your pH as much, either. There are definitely better sources for calcium.