The percentage can vary wildly depending on the brand of flavoring you are using as well as your tastes. I use TFA (The Flavor Apprentice) and normally don't go over 8-10%, while I read of most Flavorwest mixes going to 20%. I have used Inawera flavoring (Mix Mint) at 3% and it was too strong. I think one's palate gets adjusted to whatever flavoring level they use, so if you're used to 20%, dropping the percentage may seem weak for a few days, but will then taste fine. I believe that using the least flavoring you can to get a satisfying vape is the best choice, both in terms of cost and safety.