This is what I do for testers and snv's.If you're using standard 30 or 60 ml glass bottles to mix in (or whatever size bottle you choose as long as you can see the level, and shake, mix or whatever you do blend together) once you're familiar with where the fill to the level is, simply fill everything into the bottle and then add the VG to fill to the fill point. For me it's the easiest way to deal with that viscous stuff. Even if not quite as precise, it will be within 1-2% of your goal which is good enough for me.