Hard to say without knowing what the formulation of the
juice is itself. For my DIYs I use about 10% flavor and about 10% distilled water, with the rest VG and nicotine. You can ask them what the % water is, but they may not tell you, since their juices are so popular. You may have to take about 3 mL of the
juice and add drops of distilled water one at a time until the consistency is what you want. If you have a juice that is the thickness you like, you can use a small mirror, tilted, and put two drops of that juice and the one you are testing next to each other. If their viscosities are equal, they will flow down the mirror at the same rate.
Good luck!