My two cents, always use real fruit or at the very least use a portion of real fruit then supplement it with flavoring if needed. IMO, flavoring by itself tends to taste artificial. Also, real fruit tends to change the body a bit and you don't really get that using flavoring.
If you're dead set on using flavoring, pick up an oral syringe like they use to give medicine to kids. Use it to measure how much flavoring you're adding...maybe go a milliliter at a time. Usually you'll add it at bottling time, so add a little bit, mix it carefully (don't oxidize your brew), then taste a sample. Repeat until you get the level of flavoring you're after.