I would suggest, as lordmage has, to only add a few drops to avoid both losing out on your flavor as well as to avoid wicking issues that cartomizers are prone for with heavy laden VG juice.
I your going to ratio it... I wouldn't go beyond 30% VG. If you've found you've added too much, you can always add a couple drops of distilled water to gain back the consistency. If you've found that it has taken away too much of the throat hit you enjoy, you can add a drop or two of PGA/Vodka.
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