I used to use all TFA in my recipes and started experimenting with FA over the last 3 months. IMO, there is no way that you can use the same percentage of FA that you do for TFA. I have one recipe that I am converting over from TFA to FA flavors, and I've found that I've had to drastically reduce the percentages to make it taste the same. My beginning TFA recipe was 20% total flavoring with just 3 flavors (blueberry, pomegranate, and a cream). My new FA recipe uses bilberry, pomegranate, and a cream and tops out at around 7% I think. Any higher than that 7% and it doesn't taste remotely the same. I had a lot of failures with the FA until I went to the website to see what the recommended usage was--and that's how I started using them. As a general rule of thumb nowadays, if I'm going to try to sub an FA flavor for a TFA flavor, I'll cut the recipe flavor % down by 2/3 (if it calls for 10% Strawberry TFA, I'll use around 3% Strawberry FA to start). I may have to wind up adding a little more later, but at least it gives me a starting point.
And I love the "realness" of the FA flavors which is why I'm converting my favorite recipes now!