Duvets: what is the UK tog equivalent of 350g / m2 please?

[SquidgyMummy]

Hi, am in France and trying to buy a new duvet online.

In the catalogue the duvet weights are given as eg 350g/ m2, which I am guessing is about 4tog as it is the lightest weight. Probably want about a 9 tog duvet but not sure what gms / m2 to order

I have googled, but not been able to find a definitive answer as to what is the weight / uk tog equivalent.

Anyone able to enlighten me? Thanks!