Last year I started in July outdoors doing a hydroponic grow, for the same reasons as you said, and the largest of my plants yielded 218 grams. Had 3 plants and one turned out to be an autoflower variety that only got 35g, other plant got 145g. This year I started a couple of weeks earlier...
You guys are trying to be chemistry majors. 1 ml per gallon is a small amount. You have to use some simple mathematics. If you need to adjust your ph only half a point then use half the recommended ph down. approximately .6 ml per gallon. As I said it is a general rule that should get you close...
Since no one seems to know the answer, the general rule is approximately 1.25 ml per gallon of water to lower the ph 1 point. If your starting ph is more or less than 1 point away from your desired ph you will need to adjust the amount of ph down. Hope this helps.