I have just about EVERY "berry" that TFA makes--Raspberry, Blackberry, Boysenberry, Strawberry (no blueberry yet), and I can tell you that they are all very concentrated, and that 20% is probably WAY too much flavoring.
Boysenberry seems to be ESPECIALLY strong, and I haven't been able to make a juice from it with concentrations higher than 10% that was vapable, and to be honest, my next batch of Boysenberry will probably have less than 9% Boysenberry flavoring in it to get it to taste right.
I haven't noticed it etching my tanks though. That is a disturbing revelation. I've never heard of boysenberry being a plastic tank killing flavor...
I just got some Watermelon flavoring from TFA yesterday, and I'm planning to make some juice from it Sunday. I'd say the same there--probably want to start at around 8-10% flavoring, and if that's not strong enough, make a second batch with a little more.
But yeah, with TFA flavorings, 20% is probably WAY too much for most flavors.
Interesting thing about flavor concentrates--too little and your juice tastes muted and weak. But if you use too much flavoring, the juice will just be bitter, nasty and gross-tasting.
One more thing--ANY juice, made with ANY brand flavoring will taste HORRIBLE when you first make it. Shaking or stirring it by hand just doesn't seem to get the ingredients evenly dispersed, and the flavors don't really have a chance to mix and "marry". If you don't want to wait overnight (I used to let my new juices sit overnight), you need to get an ultrasonic cleaner (like for cleaning jewelry). You can get small ones for $30 or so online or at Harbor Freight. (I got the big 2.5 liter unit at my local Harbor Freight store--it does WONDERS!)
Using an ultrasonic cleaner to "shake" your juices, you can get the equivalent of 1-2 WEEKS of steeping at room temperature in about 2-4 hours time. If you are serious about DIY, an ultrasonic cleaner is ESSENTIAL, in my opinion.
There is a good thread on this here:
http://www.e-cigarette-forum.com/fo...s-ultrasonic-cleaner-part-ii.html#post7704086
and it is continued here:
http://www.e-cigarette-forum.com/fo...eeping-times-ultrasonic-cleaner-part-iii.html