*some meds are weight based but most are not.
The dose of paracetemol is 1 gm every 4-6 hrs, max 4 gm in 24 hrs ....doesnt matter how big or small you are as an adult*
That's true for therapeutic doses. The majority of overdoses are calculated on mg/kg, child or adult. In the case of paracetamol, it's x mg/kg in 24 hrs, up to a maximum of y g.
The therapeutic window is not actually that small, unless it's long term use. For short term, in healthy people, the toxic dose is a fair bit bigger than the therapeutic.
In paracetamol overdoses history is as important as dose taken. There are many drugs, including alcohol and herbal remedies, that affect metabolism, and certain previous conditions can also affect it.
And yes "drink water to flush the kidneys" is utter bollocks.
As you can see, although the op is fine, it's far better to phone 111 to talk through a full history, than ask random strangers on the internet. Even dr's often don't have much direct experience with poisonings, unless they are a&e or toxicology specialists.