How much to feed a baby

How do you figure out how much formula to give a baby? Is it based off weight? Ex:my son will be 1 next Saturday and 8 months adjusted and weighs around 19 pounds. His doctors haven't been much help so its been like a guessing game. I dont want to feed him to much but I don't want to under feed him either.

Edit: he is 100% tube fed. He doesn't get anything more than a taste by mouth or he'll choke, something we've been working on with a therapist for a few months.