It depends on how accurate you want your final PG/VG ratio to be. If it doesn't matter that much to you then it's fine to go ahead and premix it. But if you want to maintain a given ratio, you'll need to compensate for the flavorings or sweeteners you may add.
Flavorings and sweeteners are typically in a PG base, so the more you add the higher the total PG content of your mix will be. So to maintain the ratio you'd have to reduce the unflavored PG by the total of the flavoring that you've added to maintain a given ratio.
Let's say you took your nic and diluted it down to your desired level with your unflavored PG and VG. Then if you added the flavoring in effect you'd do two things, lower the nic content and raise the PG content. This is where using a calculator like eJuice Me Up comes in handy as it would take into consideration all the variables and give you the recipe for a
juice that's at the desired nic and PG/VG ratio.
Though like I said before if you don't mind that your juice is close to your desired nic level and mix ratio and it tastes good to you, then go for it.
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