You need to know how concentrated. Flavoring, concentrates are not all the same. I have a concentrate RY4 that requires 1 drop to make 6 to 10ml of eJuice. While TW's tobacco concentrates say to use 10 to 25%. Flavoring from Capella, Lorann, Perfumers Apprentice are about the same 10 to 25%. That depends of the type of flavor and your taste.
When you find out what you have, use my eJuice calculator. Don't play around and guess your Nicotine levels. The calculator will do all the work. There are recipes included as well as on-line recipes to use.