That threw me for a curve initially also. I usually just go by #of drops of flavor for 5ml of juice and go from there.
There's a great free DIY app called ejuice Me Up. Grab it here:
eJuice Me Up - Best eJuice Calculator
Under the "Tools" pull-down menu, there's a tool called "Flavor Percentage". Just plug in what % of flavoring the recipe calls for at the top (in this case 15%), and then plug in the % of each ingredient called for in the recipe (must equal 100%) then hit "calculate" at the bottom. Voila; it will give you the amount of each ingredient you need to put into your final mix (total volume).
Nice tool . I also have an Android app that I store all my recipes on, and wish it could do this, but it's easy enough to use this tool to get the % per total volume and plug that into the app.
Have fun....