There are a number of factors here, not the least of which is that each flavor is different.
I have recipes that range from 5.7% to 35% total flavor volume, but all have a fullness of taste. It is real easy to go too far with the 5.7 one compared to the 35 one, but the reason I consider them recipes is that I found the range that worked and then mix at the high-end-of-the-middle of that range.
That range is more dependent on my taste than anything else. Otherwise I'd be mixing to hit a percentage regardless of taste, which defeats the entire purpose of why I mix.
So, why do you go "too far"? Are the flavors not strong enough to you and you overshoot the range before the taste is strong enough? Have you tried the same mix over a period of time? (Some mixes change taste intensity over time).
My rule is to experiment until I'm close then cap that puppy and try it a few days down the road. I have found that if I keep adding flavor after it is close that it has a greater chance of being too much. Letting it sit means a better chance.