Don't know about potassium but I know that a reasonable magnesium dosage is 250mg a day. Magnesium should always be taken with calcium, as one without the other will leach the minerals from bone tissue. There are many blends available, usually called Cal/Mag. Dr.Mercola, the famous online nutritionist recommends 500mg calcium with 250mg magnesium. If a person is prone to irregularity, the magnesium dosage can be increased to 500cal/500mag. Avoid calcium carbonate and magnesium oxide, the cheaper forms that are not absorbed very well. He recommends calcium citrate and magnesium citrate. Also avoid oyster shell calcium.