Looking through the archives folks tend to use magnesium citrate as direct substitute for the Epsom Salts . It is taken at the same time as the Epsom Salts . If it's in powder form the same quantity of 1 tablespoon and if it's in liquid form they recommend 5oz at a time which is usually half a bottle.
They are many variations to this recommendation but it is a good starting position to find out how your body will react to the magnesium citrate.