So Centrum Has 75 Mg Of Magnesium. I Have Taken

So Centrum has 75 mg of magnesium. I have taken omeprazole for decades. My new (several months now) trainer suggests 200-300 mg per day at night, since I have been getting charlie horses in my calfes more frequently when I wake up. Have had this issue relatively often for many years long before I started an organized fitness routine. It seems that the "standard" dose available is 400 mg. WebMD suggests a max of 350 mg. Not sure what to do. Definitely need to add, but not sure how much or where to get it.... Thanks.