In Botanicare Cal-Mag Plus (the liquid), each mL contains (approximately) 6 PPM of N, 10 PPM of Ca and 4 PPM of Mg in one gallon of water. I keep my Ca/Mg at about 141/68 PPM respectively. That's about a 2.1:1 ratio of Ca to Mg. When I reduce the amount of Calcium Nitrate during third week of flower, I add in extra Cal-Mag to keep the Ca levels up.
A gram of Epsom Salt in a gallon of water will give you about 26 PPM of Mg.