21.43mg/mL if you added 1.8mL of 100mg/mL, if you added 2.4mL of 100mg/mL, you would end up with 24.13mg/mL.
At 75% PG, 25% VG, H20, PEG, or what? The added base wouldn't dilute too many flavors too much, but doubling the nicotine would inhibit a good bit of the flavoring, or *make it seem...