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For 1 drop to equal 6 mg of Boron, 2000 drops must have 12000 mg of Boron or 1.2g. But Borax is not pure Boron. 25 to 30 mg of Borax contains 3 mg of Boron.
To make the calculation simpler I will choose 30 mg. That means 1 mg of Boron is contained in 10 mg of Borax. We need 1.2 g of Boron, which is in 12 g of Borax. You need 12g of Borax per 100 ml of water. I'm not sure that much Borax will dissolve in that much water. If not, add another 100 ml of water. Then you will have to use 2 drops to get your 6 mg of Boron.
