I suppose I'd better change my example to something that is more dependent on accurate weight. I do know that at one point there were weight/dose charts for morphine administration, e.g. https://nursing.duke.edu/sites/default/files/documents/SCD-O...
but that could be an outdated practice.
The calculation charts are valid but you can give it slowly and stop at whatever dose the patient reports good effect.
I do highly recommend Prof Thimbleby's talks in this topic.