Knowing the strength (or the 'fold') of the flavor is helpful to guide usage levels. Most of what you're buying off the shelves is a single fold (single strength) - i might suggest a starting point of 0.1% on a weight basis and evaluate.
I've worked with some essential oils, however, that requires FAR less than that (as in 0.00000x%) - so little that it's hard to even measure it correctly. It's pretty unlikely that's what you're finding, but it's always good to be aware that it's out there.