What I have found is that you don't need to add any water if using the flakes. They are already approx.70-80% water, so once they dissolve they are ready to use.Just place a quantity into a bowl & let the fresh (clean)air get to them.Once melted, start applying a small quantity to skin, and rub in well.In more humid locations they are inclined to melt faster.
I have been informed by one of my practioners to add 1g of boron to every litre of solution as this makes the magnesium penetrate better & much more effective.