Check out the DIY (do it yourself) sub-forum on ECF. DIYers use flavor concentrates at between 2 and 25% depending on the concentrate and the taste of the person. At 2 percent its not going to change your nic content too much but at 25 percent it is. There is a ton of great info, and a great DIY calculator(thanks Scubabatdan) on the DIY site. The calculator does the math so the juice comes out with the right concentrations of what ever it is you are after.