Ideally doctors should be start patients on low doses and monitoring the symptoms. And then gradually increasing it to a level where symptoms decrease. I dont know about other countries, but in the UK doctors dont even consider a steady approach like this. I know you can even get magnesium via IV or periodic injections, which could also be helpful for some patients.
None of the neuro’s I’ve seen, have focused on the role of magnesium at all. They all say “take it if you want and see if it helps”.
Annoyingly when you have a blood test, magnesium levels come out normal so they brush it off and say your fine.
Just to add, I sourced my 600mg dosage from the Migraine Trust: Supplements - The Migraine Trust It even suggests that patients can take even more than 600mg.