Any flavoring you add will of course dilute the nic strength, but whether it's an acceptable final strength is up to you.
some examples to help you decide:
If you mix your nic base to 18mg and use 20% flavoring, the result will have a nic strength of 14.4mg. (18 times 80%)
If you mix your nic base to 18mg and use 10% flavoring, the result will have a nic strength of 16.2mg. (18 times 90%)
If you mix your nic base to 20mg and use 20% flavoring, the result will have a nic strength of 16mg. (20 times 80%)
Does that give you the info you need?