We don't often see much posting about our powerful sense of smell, and I'm glad you raised this point, in reference to flavor testing.
In my own experience, I've been absolutely stunned that my rather poor sense of smell has been so very helpful in making juice, particularly copying juices made by others. I can often, but not always, identify up to six different flavorings in a juice by simply smelling it. If this sounds remarkable, trust me, I felt the same way, during my journey. I've been able to discover the recipes of juices, often much later, after smell testing, in which I identified several very unlikely flavorings (rose comes to mind, as an example), only to later discover that I was correct, and remarkably so, at least from my own perspective. So, thank you for sharing this!
I often will mix up my juices using the 100DT method, which we have been discussing. This normally amounts to about 3 - 4 mls of juice, and I haven't had any difficulty using a sample that size, and remixing over and over, until I was satisfied with the flavoring, before making a larger sample. The most iterations that I have ever used to make a "final" juice is ten "adjustments." Often, four to six adjustments is probably about normal for me.
Regarding more complex mixes, this is where the fun really begins. I started by working with common flavorings....common to many juices, that is. I'm referring primarily with creams, vanillas and sweeteners (CVS). Try this example, as an experiment: compare perhaps sweet cream and sweetener to Bavarian cream and cotton candy. They are similar and very different, at the same time! Their differences, though, will be helpful in more complex mixes later on. I suggest, for the new mixer, to use, perhaps, 4% of each flavor and compare the two samples. After experiments with, perhaps, three different creams and sweeteners, add a vanilla (CVS) to the mix. Knowing the right creams, sweeteners (if any) and vanillas (CVS) for a particular flavor target will enable one to mix dozens, perhaps hundreds of different mixes together, all with different flavor results.
I did this myself, and have repeated these experiments many times. Knowing both the original single flavorings and the more complex CVS flavorings has been key to my own mixing development, and I can recommend these experiments for one's own mixing journey. First, the individual flavorings (I have a chart I use for starting percentages in the dropbox (Flavoring Chart), and, then later, adding the CVS flavorings to the arsenal, will put one leaps and bounds ahead of the game! I tend to think of each flavoring and each CVS combination as arrows for my quiver! The more arrows in the quiver, the better my arsenal, so to speak!
As we each have our very own taste and test sensibilities, what works for me, may need to be adjusted or changed for you. In the end, they are all just potential and possible suggestions for use by each of us, with no guarantee as to accuracy, or even adequacy, either way. We can and do learn from each other. However, much of this journey is learned by trial and error in our vape labs, and, often times, there are differences in our perceptions, so we do find ourselves often groping in the dark for the light switch....the answer to the question, the missing puzzle piece that can prove elusive, from time to time. There is no substitution for experimentation and trial and error. Give it a go, my friends, and Good luck! You will get there in time. Be patient and diligent. It will come...often uniquely, in its own way, for you! Best!!