Here's a quick way to get the thickness about right:
Take a small mirror and clean it with plain tap water and dry it good. Now take one drop of your uncut e-juice (just the way it came from the dealer) and put one drop on one side of the mirror.
Now mix up 9 parts of VG to 1 part of distilled water or 100 proof or more of alcohol, like Vodka (I prefer the water because alcohol will give the mix an alcohol taste and more throat hit). For example, if you wanted to add 5ml of VG, then mix up a batch of 4.5ml of VG and .5ml of water (this is your 90% to 10% ratio). Now put one drop of this VG mix beside the other drop of e-juice on the same side of the mirror.
Now turn the mirror so both drops are away from you and tilt the mirror up at about a 60° angle (not quite vertical) and watch both drops flow down the mirror. Knowing that the original e-juice is the right consistency you want the VG mix to flow at about the same speed down the mirror. If it's about the same, then use the 90/10 VG mix to cut your e-juice and you should be all set. If the VG mix is slower, make that mix 80/20. If it flows faster make it 95/5. I'm sure you understand what I mean.
As long as the mix you are using to cut it with flows the same as the original mix, then you know that you've pretty much got the viscosity (thickness) about right. Before long you'll be able to tilt any mix in a bottle and tell that it's about right.
Hope this helps.
