Mt Baker suggests starting at 15% but I personally would start at 10% because it's easier to add more if needed then try to take some out if it's overflavored. By the way your flavors will normally be in a PG carrier so your
juice won't be 100 VG. That's ok though because 100 VG would probably be too thick to wick properly. If that is a problem you can thin it out with a little distilled water. Good luck with your DIY!
Cheers,
Steve