Colleen, excellent information of where to review magnesium.

I pulled this from A.I. Microsoft Co-Pilot. A.I. listed where they got information from. I think important that it mentions diet as many will be getting more magnesium than others. I do see and what I mentioned before is the difference in magnesium recommendations are based on sex and age.
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The recommended daily amount of magnesium depends on age and sex. These values come from the U.S. National Academies of Medicine (Food & Nutrition Board) and apply to total daily intake from food + supplements, not just OTC supplements.

Recommended Daily Magnesium Intake (RDA)
Adult Men
19–30 years: 400 mg/day

31+ years: 420 mg/day

Adult Women
19–30 years: 310 mg/day

31+ years: 320 mg/day

Women Who Are Pregnant
19–30 years: 350 mg/day

31+ years: 360 mg/day

Women Who Are Breastfeeding
19–30 years: 310 mg/day

31+ years: 320 mg/day
Important Notes
These numbers are general population guidelines, not personalized medical advice.
Many people get 200–300 mg/day from food, so supplements often fill the gap.
The upper limit for supplements alone (not counting food) is 350 mg/day, unless a clinician prescribes more.
Different forms absorb differently (glycinate, citrate, malate are well‑absorbed; oxide is poorly
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With me my Electrophysiologist recommended I take 400 mg of magnesium citrate as is so easily absorb. With some though citrate can cause some digestive issues. I asked my EP about that and his second suggestion would be glycinate as also very absorbable. I actually do both using both citrate and glycinate to reach my 400 mg.

I mentioned the blood test done (don't think a more accurate test was done) at hospital and it came back my magnesium was lower than 2 (again not sure what that means but what nurse told me). Thus they set up an I.V. with magnesium in it as they would not release me until it was over 2.

I take my 400 mg separated over the day just like other OTC supplements I take.

I would strongly suggest if you can to do a pharmacist consultation. I saw this offered at Mayo Jacksonville and set up an appointment. It lasted 45 minutes. The pharmacist went over every single prescription and supplement I take. She had an assitant who would input into a computer and then discuss the pros and cons for me and if any contradiction with medications and supplments. She went over when to take for each one.

If you ever get a chance to do this with a pharmacist (mine was at Mayo) it is well worth the time. I came off so many of my OTC supplements. Pharmacist explained it could affect a specific (she would list it and why) medications I was taking. Never done this before and glad I did.

The Mayo web site that Colleen added is an excellent article on magnesium. I think important to always advise your doctors of what supplements you take. On MCC I was hearing how much potassium was helping those with PVCs. I decided would try it but I follow my doctors recommendations to check with them first.

I contacted my heart failure and EP and both said we do not recommend you take a potassium supplement and explained why. Stated your blood test shows you are at the high normal limit now and do not recommend you take a supplement.

So you can see always check with your doctors. I wish I had a diet were I was consuming magnesium the natural way through diet. But I know I don't and in my case got a specific amount and type form my EP.