This is how I do it without calculators.
Every ml of 100mg liquid raises the nicotine level of 30mg by 3.3mg.
So 1ml is (6 + 3.3) = 9.3
2ml is (9.3+3.3) =12.6,
3ml is 15.9,
4ml is 19.2,
So guestimating around 3.75ml does the trick.
How did you come up with this...
"Every ml of 100mg liquid raises the nicotine level of 30mg by 3.3mg"