Perhaps someone else may have better info for you because I have never used liquid pH adjusters. I have always used the dry crystals by earth juice. But as you must have guessed already, manufactures cant give specific directions because of all the variables. Like the amount of solution needed to be adjusted, as well as how far off the pH is in the first place. You can even get technical and the type of
nutrients and weather the nutrients have buffers in it or not (as well as what type of buffers) all make a difference.
Though my recommendation would be start by just adding a couple of drops, mix and wait a couple minutes, then re-test and repeat if necessary. It will take some trial and error, and you may even wind up adding to much and needing to go the other way (happens to me still from time to time). But it wont take long before you get real good at judging how much to add. It's kind of like saying add salt to taste. My best advice is it's better to add to little than to much, especially if your not familiar with the product. But it does take some trial and error to get used to.