I have another question for the experts
I want to make this recipe
The base is 50/50 3mg
Flavor shot is 9.0% pg
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If your base is pre-mixed 50% PG and 50% VG with 3 mg nicotine, you won't end up with juice that is exactly 50/50 once you add flavoring to it, unless you have separate PG or VG to add at the end.
With pre-mixed base that contains nicotine, you put your flavoring in the bottle first, which in your case when using the one-shot would be 9%, or in a 100ml bottle, 9ml. Then you fill the bottle up nearly to the top with your pre-mixed base. As
@88ArDeN88 said, your result is approximately 59% PG and 41% VG, because your 9% flavoring is carried in a PG base. That's all you do. Otherwise there would be no point to buying pre-mixed base.
If you want to be more exact than 59/41, you'll need separate VG, 9ml of it to be specific, if you're mixing a 100ml bottle. That's going to be about as close as you can get to a final mix of exactly 50/50. Put your 9ml of flavor into your mixing bottle, then 9mg of VG, then fill with pre-mixed base.
Beg pardon if I've misunderstood your situation.
ETA: This is not right, I goofed. Correcting...