Adding an 18mg bottle of freebase nicotine to a 50ml capacity bottle of e-liquid will mix to a 3mg/0.3%. To mix to a 6mg, you’ll need double the amount of nicotine.
Some shortfill bottles have enough space for two nic shots, otherwise you'll need to decant some liquid first to allow for the extra bottle of nicotine, then mix. By
the same principle, if you want a very light nicotine level, just put in half the nic shot to your liquid.