Here's a method I'm trying out, for mixing two flavors: First, work out the ratio for each of the flavors separately. Then make a batch of each, say 5ml. So, if I wanted to create a 'peach iced tea', I would mix a 5ml bottle of peach (8% flavor plus tartness, to taste), and a 5ml bottle of 'black tea' (5% flavor, plus sweetner, to taste). Each of those should be vapable on their own (add sweetener, tartness, as desired). Also, I like to let them steep overnight, or at least for an hour or so. Now, its time to mix these 2 together, in small batches (1/2 to 1 ml). 9 drops peach, 1 drop tea. Shake, let steep (flavors need a little time to blend themselves). Taste, and take notes. 10 drops should be enough to tell if the flavor balance is what you're looking to obtain. Clean atty and let dry while making the next mini-batch: 8 drops peach, 2 drops tea. .... continue until you've tasted the 1 drop peach, 9 drops tea.
By starting with flavors that are vapable by themselves, this mixing method should get me at least *close* to the proper balance between peach and tea, and not produce a totally terrible mix at any point in the process. Let's say I liked 4 peach to 6 tea, but found it weak. 4/10s of 8% peach is 3.2%, 6/10s of 5% tea is 3%. I'd mix up my 'peach tea flavoring' with this new ratio, enough for a 10ml batch of e-liquid, and then work out what percentage of this I need for the flavor I want.
In theory, this should get you close to the ratio you want, without a lot of wasted batches. I do my testing 0-nic, just in case I do have to flush a batch.
