Here is my formula for re-mineralising RO or Rain water:   Powdered compound Grams needed: Calcium Sulphate CaSO4 Heptahydrate (so it dissolves easily) or Dihydrate if you can't find it, but dihydrate is not as soluble. 58gm Magnesium Sulphate (Epsom Salt) MgSO4 37gm Potassium Sulphate (aka Sulphate Of Potash) K2SO4 11gm Iron Sulphate