I make Magnesium Oil quite often as not only do I use it myself but I also use it for my horses too.They get it in their water bowls.However I make it from the concentrated food grade Magnesium CHloride Powder as that is the purest & also the most concentrated.
If you want to make it from the flakes it will be a lot less concentrated (& possibly not so pure)as the flakes have a lot more water in them.
The best way is to just leave the flakes in an open bowl & they will melt by themselves quite quickly. Collect the liquid & use it direct. If you are going to add them to water please use purified water & not just ordinary tap water as any impurities in the water will be absorbed with the magnesium and that's not a good thing in my book.The rule of thumb is 1 cup of flakes or powder to 1 cup of purified water.It is measured by volume & not weight.
So if you want the strongest Magnesium Oil mix use the Powder or let the Flakes melt. Hope that helps.