The problem with mixing up the magnesium yourself is that you don't know if you are getting the ratio correct. It is my understanding that the proper ratio can have a lot to do with the rate at which your skin absorbs the magnesium.
Regarding the small amounts of magnesium chloride left on your skin after drying, Im told that this is natural and too be expected, and is not a sign that you haven't absorbed the correct amount.
I use DermaMag from Magnesium Direct. As far as I can tell, they are the only bottle to carry the "Zechstein Inside" logo, which means you are getting the authentic Zechstein magnesium oil, not some counterfeit.