I use Source Naturals mag malate 1250mg. I have no idea what the 1250mg means... has nothing to do with the amount of elemental magnesium or the amount of malic acid. I keep meaning to email them and ask.
I would not start out that quickly. I would start with 1 tablet morning and night. If your stomach and bowels are tough, then maybe 1 in the morning, 2 at night for 425mg a day.
My doctor had me start at 400mg twice a day and it KILLED my stomach.... omg the pain and vomiting.
How fast your work up depends on your gut. I didn't notice any significant improvement until I was at 600mg for a couple weeks. Adding a magnesium oil (made from magnesium chloride flakes) and using it several times a week will go a long ways in improving your magnesium deficiency.