Here's the foolproof way to calculate nic percentages.
TOTAL nic in the bottle divided by total volume.
SO - you've got 7mls X 18 mg/ml = 126 mg of nic total
When you fill the bottle with no-nic, you've STILL got 126 mg of nic, but now it's in 10 mls, right?
So that's 126/10 = 12.6 mgs/ml
Make sense?
Works for any mg/ml calculation...