Hi bam I always start out small 6 ml and I always leave room in the bottle in case I need to tweak I can add more
juice or vg whichever is the case! I also find the 10 or 25 ml cylinders are a good tool to mix in. You can put a pipette in and draw some out to taste, If you need to add more flavor or vg pg there's plenty of room to do so. Good luck let us know your creations and how they turned out