Hiya all! I知 Noora and I知 totally new here ☺️

I知 also very new to silversmithing and would love some advice please.
What is the best way to determine the correct UK ring size if you have
the circumference of the finger? My friend gave me a circumference of
55mm which I thought would be size O. However the ring ended up being
way too big 🙈 I only then realised that different charts give a different
size for the same circumference, so what should I do to get the correct size?
Is there a magic chart that is the correct one?

Thanks so much,
Noora