The percentages in recipes are their proportion to the total amount of liquid. So, if you're making 100mL (not recommended until you find recipes you like) then 8% would be 8mL of that flavor as part of the total volume, including plain PG/VG/Nic
Get hooked up with the e-juicemeup calculator
http://ejuice.breaktru.com/
As you read recipes, you can see how the different flavors combine with the levels of PG and VG and nic to get you what you want in a recipe. I basically mix "max VG". My flavorings are PG-based, I top with nic to my level and then plain VG to make up the remaining volume of my mix. I'm not real scientific, but over the past 14 months, I've only mixed up about 100mL total that I just couldn't do anything with. I will mix about 150mL at a time of different juices when I sit down for a mixing session (about once a month).
If you're unsure of something, do it in a small batch (<10mL) so you're not apt to waste too much if it's too heinous