I did 1 1/2 tsp for half a gallon and it dialed the pH down to 6.5 (read the meter a day later) so I'm thinking it was a perfect amount. I also tried feeding molasses and vinegar in the same night and my plants seemed to respond really well, though maybe it has more to do with the pH not being 7.5 anymore so the nutes aren't locked out from the molasses and other nutes that were just sitting in my soil. Overall if you're like me and cannot see spending money on a ph up/down regimen apple cider vinegar is a great way to go, gonna feed with it once a week until my girls are done (doing molasses every 3 days until harvest). I just don't get why people tell other people on the forums to mindlessly spend money on something you can get from an organic source...as well as something you can find in your house.
Also I might add I've been trying to find a more organic way to grow, though I'd never adhere to organic growing because of all the corporate BS that surrounds that word in the growing world...