Actually, table salt and chalk work out right, baking soda is close. The two numbers add up to 264, as they should. 1gm/gallon of anything = 1000mg/3.79liters = 264 ppm

If your salts split up in solution then the ratios of the two components will scale with their weights, but the sum should come out to 264ppm for 1 gram in 1 gallon of water.