You have to know how much ELEMENTAL magnesium there is.
Forms such as oxide will say it's 500 mg, the concentration is high, but it's also potentially laxative. Same for citrate.
Look for magnesium glycinate, bisglycinate, malate or threonate. Threonate is more expensive and possibly not necessary depending on use.
You can shoot for 200 mg of elemental magnesium to start and go from there. Hardly see why there would be a need to go above 400.
