When you circumvent the skin barrier with something like DMSO....you run the risk of getting TOO MUCH of the magnesium into your body. I wouldn't begin to know how you could make this without proper measurement of the magnesium flakes and water, to get a safe concentration to mix with the DMSO.

The skin is a very effective barrier to avoid toxins and poisoning.
Epsom salts baths need time to work, and even then the amount of mag that gets in is still quite small.

Any contaminant in the magnesium you use to make the solution will also go into the body with the DMSO. And there will certainly be other substances in that product you are using. Food grade is not 100% pure....it is just cleaner than non food grade.

Just a quick look around Google, and the flakes are not cheap either. My magnesium lotion lasts me MONTHS...about 3 months a bottle. This is because you only apply a small amount...the lotion carries it right thru the skin barrier.

Sometimes do it yourself products may backfire on you!