When you say with 18mg nicotine do you mean it came with a 10ml nic shot that you added to your liquid?
The best setting will depend on the individual as we all like different settings. Start with your watts low and take a draw, turn it up by 5w and try again, keep going until it works for you. As long as you don't exceed the max wattage for the coil you'll be all good and find what works for you.