I wanted to note this concise article on acid-adjusting juices:
The article includes this handy reference table:
Grapefruit to lemon: 40 grams citric acid powder
Grapefruit to lime: 27 grams citric acid powder and 13 grams malic acid powder
Orange to lemon: 52 grams citric acid powder
Orange to lime: 32 grams citric acid powder and 20 grams malic acid powder
Pineapple to lemon: 52 grams citric acid powder
Pineapple to lime: 32 grams citric acid powder and 20 grams malic acid powder
As the article mentions, Existing Conditions practices acid adjusting a lot. Dave Arnold also covers the topic in a more roundabout manner in Liquid Intelligence.
Alas, it’s pretty hard to scale this down for the home bartender. Gram accurate scales are cheap, but precision is probably less than 0.1 g. You have to be willing to make squeeze at least several ounces of juice to keep this practical.