Recommendation for magnesium supplements is to take 1/2 the RDA for them. Studies have shown up to 70% of Americans are low in magnesium because of the diet. In fact many people are walking around low in magnesium and THIS is a risk factor for long QT syndrome, esp those taking drugs like methadone, and Nuedexta. People on long term Lasix will lose magnesium daily thru the urine and so do diabetics or those with insulin resistance.

You can choose to get your magnesium from foods.
My thread here illustrates that:
http://neurotalk.psychcentral.com/thread1138.html
Most people do not eat foods that provide the daily RDA for magnesium.

Potassium on the other hand, should not be supplemented without a doctor's supervision and testing.
Many foods are good providers for potassium. Today recommendations for food and potassium are 4500milligrams a day.

To check your food choices for magnesium content or potassium content this site is very helpful:
http://nutritiondata.self.com/