Each tab isnt 400. That tablet would be too big to swallow. lol. If you look at the serving size then the amount of magnesium and divide that, you'll get the amount. I'm not sure what the 1250mg refers to. For the malate you need 3 tablets to get 425mg magnesium.
I like to start slow- around 200mg morning and night. My dr suggests starting at 400mg twice a day but that killed my stomach which can be sensitive. Malate is best taken in the morning and glycinate at night.