Potency differs not only in between brand names but even for flavors of the exact same brand. Companies like Flavourart as well as Inawera are notorious for their potent focuses, where 1-2% is more than enough to dominate a recipe-- yet even a few of their own can be made use of in the 5-10% array. A good sign of strength is the "ordinary mixing amount" portion, which can be located on each flavor's web page at ELR.
http://jaredwcir798.jigsy.com/entries/general/exactly-how-to-begin-making-my-own-e-juice-