I mean any medication requires trial and error. Colloquially therapeutic usage would correspond with typical medically prescribed doses which enable people to function at a proper and normal level.

Though it could also be considered therapeutic to take just enough of a drug to push someone in the direction of a certain level of functioning. Therapeutic refers to doing/taking something to heal a disorder, and technically a disorder can theoretically be defined as functioning at a lower level than intended. In this way, therapeutic use is the gradual use to push someone in a desired direction of cognitive functioning, typically without the major crashes and such that you would get from recreational abuse.