Just got a reply from my sister on the dosage for magnesium. My doctor has me on 500 mg for my migraines, my sister found that taking 5 times the daily recommended dosage made her migraines responsive to OTC medicines where prior they had lost their effectiveness. She did not continue to increase the dosage for complete control of the migraines, but found measurable relief while on the high dosages of magnesium. I have read online, since originally posting this and have found migraine patients taking doses from 250 - 1600 mg daily, with significant results.
I would strongly recommend talking to your doctor about the correct dosage for you. Also magnesium can cause diarrhea, so it may be smart to start at a lower dosage and work your way to a larger dosage if that is your intention. I am not a medical person, am just relating personal experience.