How much Sodium metabisulphate per 4L-ish of wine?

[WishboneBrewery]

Can someone tell me how much Sodium metabisulphate to use per 4L-ish of wine?
Fermentation looks to have all but stopped in my Sainsburys Red grape, so I figure its time to stop it..?
And I was planning to use bentonite as the finings, so I presume its best to stop fermentation completely before adding this?

cheers for any help with my fledgling brewing questions

[Aleman]

The normal dosage at racking is 1 campden tablet per gallon . . . If this helps

[WishboneBrewery]

so I guess that its about an 1/8 of a teaspoon??

[edit1now]

More of a teaspoon handle - scoop up a little pile about a centimetre long by 4mm high by the width of the handle, judging by when I've been measuring metabisulphite with the digital scales. 1 Campden tablet = 0.46g of metabisulphite.

[WishboneBrewery]

excellent, cheers for that

[WishboneBrewery]

I think I might have picked a big teaspoon handle, there is a odd fizzy sensation when tasting the batch that had the Sodium Metabisulphate in it rather than a campden Tablet...
Will this gassiness disipate if the bottles are left to breath????