The Canadians are awesome when it comes to health information. I was impressed with that link by Canadian Health department.
The calculations by jam1 and the link you have posts seems very convoluted and illogical. I will make it simple. If I am wrong please correct me.
1 teaspoon = 4 grams = 4000 milligrams (mg) which means
1/8 teaspoon = 0.5 grams = 500 milligrams (mg)
Borax has 11% Boron (which is true in jam1's post) so
500 mg Borax X 11% (.11) = 55mg Boron (in 1/8 tsp)
So let's say you were drinking 1 liter water with 1/8 teaspoon Borax, you would get 55 mg Boron. But if you drink only one cup (8oz) of that water everyday, which is approximately 1/4 part of liter then you only get 13.75 mg Boron per day.
1 liter = 33.8 oz
55 mg Boron / 33.8 oz
= 1.627 mg Boron per 1 oz
If you desire to take 20mg Boron per day, then you can drink 12 oz
of Borax water per day.
I hope this helps.