I use 20-25% flavoring and sweetner combined, but then again it depends on the individual flavor and how strong or sweet it is. Use your nose and try and practice, you'll get the hang of it soon enough. I was pretty confused on the flavoring issue as well and run into hic-ups along the way, but I keep trying and I think I've been pretty sucessful with my juices I've created. You can always start small then add more, or if its too strong flavor you can dilute it out with vg/pg or distilled water too, just be careful with the nic juice if adding more. I only vape between 2mg-8mg nic anyways so I don't mind if I dilute a flav out a bit. Have fun! 
