Yep, and that's only using the rough metric of bodyweight. There are a number of other major factors like size and health of liver, whether the patient has any under/over expression of metabolic enzymes, etc that can further complicate the determination of proper dosage.

Would be super cool if we had an objective measure of proper dosage - such as blood concentration or excreted metabolites - to give us more insight into how the drug is being metabolized by the individual.