For single flavors with Capella, I'be been using 5%. Higher total flavorings result when using multiple flavors, for example instead of 5% one might use 4%+3%+3%+1%=15% ... a little less or the same of the main flavor, plus other flavors and sweeteners. If you got a single flavor that's more complex, the manufacturer might correct for that and make it so the flavor contains more stuff and you only need 5% again.
Another symptom of too much is when dripping and you switch to the flavor it's really good, but as it takes over from the previous flavors it gets too much and nasty.
Another symptom of too much is when dripping and you switch to the flavor it's really good, but as it takes over from the previous flavors it gets too much and nasty.