Women need about 300 milligrams per day. You'll get some of that just eating regular stuff. I do take a Mg supplement as I've been having some odd muscle twitches these past few months. I take about 800mg a day as the bones themselves store Mg and if your blood becomes depleted it means your bones are depleted.

Note that you have to be very careful when reading the labels. If you take 300 mg of Magnesium Oxide then you're only taking 24/(24+16) *300 = 180 mg of Magnesium as they count the O (Oxygen) in that 300mg. Plus MgO doesn't break down that well in the stomach and will send you to the bathroom frequently.

I take the Mg supplement sold by GNC called Super Magnesium. Your "Potassium & Magnesium 40mg" seems almost pointless. Sorry. My Potassium I get from No Salt, about a teaspoon a day. The "Calcium Magnesium & Zinc 400mg" sounds better but idk how much Mg is in there. What does the label say?

And no, Mg won't kill you. Potassium OTOH...... will hurt you in high enough (very high) amounts.