Yes in your example, even your base is 50/50 12mg, you will not achieve 12mg in that 30ml when you add 10% flavour. If your base is either PG plus 12mg or VG plus 12mg, it will be worse or less mg in that 30ml.
If you get a PG base 36mg, then the mix for your 12mg in 30ml (50/50) would be:
15ml...