1/2 oz (or 1 TB) to 1 gallon of water is pretty popular. There is no exact amount, but you just don't want the chickens to stop drinking because it is too strong. If I put a 1/2 tsp in a glass of water, it tastes too strong, but a 1/4 tsp tastes just right.
Yes, many of us use it in our flocks every day. Just don't use it with galvanized metal waterers, since it will rust them and may even cause toxins to be released. Plastic waterers or even stainless steel are okay.