I think it depends. I weigh 128 pounds. I take mag citrate tablets every night, usually three hundred mgs, sometimes 400mgs. This helps contribute to a large soft elimination every morning. I'm really regular and don't have any bowel problems to slow down the "stonal exit."
I don't use extra mag citrate for flushing, since I've had leftover Oxypowder capsules, I usually use those or milk of magnesia the next day. I don't need much help though. Your mileage may vary. I'm thinking if 300mg gives a laxative effect and 400mg is a recommended daily dosage, depending on your size and bowel state, you might try 400 to 600mgs. If the amount of magnesium sulphate recommended by Dr Clark isn't overdosing, then I should think you could take a large dose of mag citrate without problems. For a short period like a flush anyway.
I hope somebody with more info will answer this for you! Good luck and let us know how it goes!