What is the optimum ratio/percentage of fluoride in water?

0.7-1.2 ppm. The optimally fluoridated water is safe to drink, does not cause systemic disease and provides the best protection against dental caries. Ask your dentist for more information.
0.7 ppm (mg/l) Fluoride in water is not measured in ratio or percentage but in parts per million (ppm) or milligrams per liter (mg/l). As of january 2011 the cdc has set the optimum amount of Fluoride in drinking water at 0.7 ppm.
.7 parts per million. The traditional range was .7-1.2 ppm. It takes very little Fluoride to have a very profound benefit.