Let's say I don't trust that anecdotally, how can I measure mineral content and get the right amount? There are plenty of studies that point out too much Magnesium can give people a heart attack.

"Symptoms of magnesium toxicity, which usually develop after serum concentrations exceed 1.74–2.61 mmol/L, can include hypotension, nausea, vomiting, facial flushing, retention of urine, ileus, depression, and lethargy before progressing to muscle weakness, difficulty breathing, extreme hypotension, etc..."