It depends on the flavor and the palett. Some manufacturers flavors are weak, while others are super or ultra concentrated. When mixing flavors for the first time a good rule of thumb is to start low and add more flavor as needed. I start at 5% when mixing super or ultra flavors and 10% for regular flavors.
I concocted a flavor that is just like an Altoid. It's strong at 20% of base flavor, with an additive to waken up your nasal passages.