I have not used any sweetener for a couple of years now, but here's what I've done in the past:For me sweet is 5 drops of NuStevia liquid per 120 ml bottle of ejuice. Really sweet is 10 drops.
I used more than Robert does, as much as 10% in fact. Anywhere from 1% to 10%. I know that's odd coming from somebody who doesn't add sweetener at present.
The thing I like best about liquid stevia is that for me, it keeps getting sweeter when you add more. Sucralose seemed to me to reach a saturation point where it stopped getting sweeter. Also, stevia in vapor does not have the slight off-taste that made me stop using it in my coffee.