Correct. Your50%PG/50%VG 12mg/mL nicotine "base" would be vapeable right out of the bottle.
The 99.9% nic is listed as an ingredient used to make the finished base. They could use any strength/potency to arrive at the final content and it would have the same end result.
What does affect
your final mixing results is what you add to that base. In this case; you added 10% flavoring (1mL of flavoring added to 9mL of base). This will dilute the nicotine content by 10%. 90% of 12 equals 10.8 (mg/mL).
It will also alter the final PG/VG ratio a little.
If your flavor concentrate is 100% PG (most common(?)) you end up with a new ratio of 55%PG/45%VG.
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No. As I mentioned above; the concentration of 12mg/mL "base,"is not any different than 12mg/mL "ready-to-vape" e-liquid. That is really what it is after all (PG, VG, nicotine).
Fumes are not a concern at that level for
most people (I imagine there are always exceptions). Like
any e-liquid product; you want to use caution and keep it away from children, and pets. Be responsible and keep it safely stored.