My ADV I mix at 8% total flavorings. But it really depends on the flavoring I'm using. Some are much stronger than others. There are mixes I make that I use much more than 8%, and there are mixes that I use much less.
In fact there's this one very specific flavor I use that I mix at a fraction of a percent (by itself) for a 30 ml batch; it's that strong.
Start with a low percentage of flavor. I generally start a new flavor at 5% (that's all flavoring combined, not individually, if you're using more than one flavoring), and then increase as needed. Most of the time I need to increase the flavoring, but I find it easier to add more flavoring than dilute an over flavored batch.