It doesn't really matter...whatever amount your birds will tolerate. I never measure...I just tip the jug, allow a glug to flow and that's it. One glug for more water, a little glurp for little waterers. The thing is, it doesn't take much vinegar to make a difference and any is better than none. You don't want to put so much that they won't drink the water or you see their beaks dissolve when they dip for a drink! Just kiddin.... won't happen.
I'll tell you how I make more unpasteurized ACV right now. Before I had an orchard and could make my own but right now I don't have that, so I just buy regular store bought vinegar and add some of my Bragg's to it, leave the cap off and place it in a dark, room-temp place. Within a week you will see the swirl of the mother starting to build in the new jug. If you don't, add some apple juice to it so that the cultures have some sugar to get a boost and it will happen. I've never had to add the apple juice. When you see a new mother forming, you can cap it and store it somewhere out of the sunlight.