I think it'd be better to start low and add more if you need it, you can't take out afterwards
Example, I mixed BYO Nutzilla at 10% as that's what I used on my other F'Art flavours, and although it's mellowed now (or could be a bit of vapour's tongue), when I first mixed it and used it, it was like being smacked in the head by a sack of hazlenuts lol.
So, different flavours may well differ in strength you want
Think 20% is too high for most F'Art concentrates ... you might find this German spreadsheet useful as a broad guide.
Select Flavourart from the list of vendors at the top. I would normally check this for a starting point if there's no guideline from the supplier.