I'd suggest that you start closer to 10% flavoring then adjust fire accordingly. Also, the drops per ML needs to be adjusted based on what you are dripping from (good for when you are adding multiple flavors).
I was telling another via PM earlier: If you want a general sugar-like sweetness, use ethyl malitol. If you are going for citrus, tart, or tangy flavors, use liquid stevia extract. I use alcohol-free stevia @ 5 drops per 10ml, but it is pretty potent. you may want to start at 3 per 10 and adjust to taste.