Been reading on magnesium and found that those blood test they typically give you are worthless, maybe less than worthless - says Carolyn Dean, MD, ND(author of The Magnesium Miracle) and many other experts on magnesium.
Those tests test the serum which is the amount of Mg in the blood. The blood contains only 1% or less of the Mg in your body. The amount in your blood can be okay, but way deficient in your whole body. 99% of Mg is in the cells (bones and tissures), not the blood serum. 90 some % of Dr's are clueless about this. Even most Cardiologists are formerly mis-educated about Mg deficiencies and Mg supplementation.
Here's one of the best pages on this topic.
http://www.livingwithatrialfibrillat...-test-results/
One person had been taking 750mg of Mg for about a year, and WAS STILL LOW and Mg deficient!

Perhaps she'd been taking Mg Oxide, the cheap and non too well absorbable kind - that is in most pills. Not much of it gets absorbed in the body. Anyway, her REAL Mg test showed she was still deficient. Even this person's Doctor said her Mg levels were okay, when he looked at her serum test.

You may have been taking some form of Mg, say 800mg, and only 400mg is absorbed.
Another person, and many people, are eliminating 400mg of Mg in their urine alone. That's probably more than most people are taking in! And that's a tad more than the RDA on Mg.
It goes on to say that the symptoms of Mg deficiency is a much better way to decide whether you're low or not.
Personally, I'm taking mostly Mg Citrate and will continue to take some of that, but after researching, will start taking some Mg Glycinate (more is absorbed) by Bulk Supplements. Several articles say that it may take up to several months to get you body to build back up the right amount of Mg; so be patient. That article above says/implies one probably shouldn't take more than 800mg to 1000mg, depending on your weight. (also you don't want to take this if you have kidney disease or severe heart disease) BTW, they push a supplement called ReMag in that article, probably because it was formulated by Dr. Dean. Am sure it's a great Mg; there just might be less pricey forms.
Other articles say that just because you're suddenly getting loose stools, doesn't necessarily mean you're getting too much Mg. Just take smaller doses, say 2 to 3 times a day. They say don't go over 350 mg per dose. Note: you should take an extra 5000 IU of D3 along with your Mg supplement.
What a great article!
edit: on another note, other things determine how much actually is assimilated (used) by your body. 1) The Form - Mg Oxide may say 500mg, but only 30mg may get used. 2) Your age matters - the amount actually used at age 70 is only around 70% or less, as that of age 30. 3) with food or without food - If you take it with food, definitely a different amount will get used as opposed as taking without food.