This was according to the Arthritis survey; he used 30-60 mg borax, but how much elemental boron it is in 30-60 mg of borax I dont know. Sorry if I mixed up these two; I did some extra reading yesterday on borax-boron;
Elemental boron is not found in nature; it is bound to borax or other borates (minerals) small doses in water / soil. Safe dose for boron seem to be 3-18 mg daily; but it is difficult to find data for this. Our body consist of approx 18 mg boron.