I confess we use slimline tonic, I don't like the taste of sugar. For two, add 50 ml of gin, and then split one little can (I think they're 125ml) between them, I think that's about half and half. It's some kind of crime against humanity to put a splash of gin in a glass and add about 250 ml of tonic. If I'm paying £40 for a bottle of gin, I want to taste the bloody stuff.