I'm actually vaping the Watkins Root Beer right now and it's pretty good. It's steeped for about 36 hours and has had a little time to mellow out, so I'd almost recommend going with 20% instead of 15%, but anything more may be a little overpowering. It just depends on how much you like Root Beer

.
So with that in mind, 20% flavor and the remaining 80% would be your PG/VG/Nicotine. If you wanted to do a 10 ml test bottle, this is how you'd mix it. This is based on diluting a 36 mg / ml Nicotine (100% PG w/Nicotine) solution and a 65/35 PG/VG mix.
- 4.17 ml Nicotine Juice
- 0.33 ml PG
- 3.50 ml VG
- 2.00 ml Root Beer Flavor
Shake for 20-30 seconds with the cap on, allow it to steep for 24-36 hours to start, test vape and see how you like it. This will make a 10 ml bottle. It should turn bright yellow, but that can depend on your temperature and air settings. I leave mine in a dark cabinet dedicated to just juice.