@Punk In Drublic — you have received good advice from the other mixers. I am around ± 10%. I have mixed recipes that were < 5%. It all depends on who created the recipe and the brand of the flavors used for me.
New recipes I only mix a 10ml bottle. I will try some at the appropriate time. If it is something that I am enjoying I will continue sampling it every seven days or so to see now the flavor changes until the 10mls are gone. If it has faded over the course of testing it, I may up all flavors by 50% especially if I plan to mix a larger bottle although I do not do this often. Some flavors are known to fade and lemon is one of them and of course is my favorite flavor. This has been cured with FE Lemon for me.
I always have multiple set ups going at the same time so whatever I mix the flavor needs to hold up. I was told a while ago by a juice maker that the total flavorings in a mix is anywhere from 25 to 30% however take this for what it is worth. I have been vaping since June of 2013 but only started DIY a little over a year ago.