If it helps at all, I got some *** white & blue flavour concentrate from Nicvapes with my last order (which I assume is Flavorwest) and it's been the most pleasant red bull type that I've come across so far.
Having said that, I've got a small batch of DV red steeping at the moment and it smells lush.
As to concentrations, that's quite dependent on what sort of flavour one is mixing. I prefer fruity type flavours and being a total flavour hog, I tend to mix around 25, sometimes 30% flavourings... but I'm a bit of a wierdo so most folk probably wouldn't use concentrations that high. All comes down to what works best for you.
If I do a tobacco type mix (which isn't often) that percentage drops right down to between 5 and 10%.