I'm on Detroit water and just checked the latest water test I could find -- .57ppm fluoride. If I did the math right, there is 1mg fluoride in every .46 gallons of water.
Additionally, the maximum level allowed in my city is 16mg per gallon, and has been since I was born. However, in the year I was born(1992) the maximum measured level per the city test was 48mg per gallon.