Howdy all, I just did a rough and ready test, have some 50/50 pre-mixed, so I used nine drops of that, to one drop of spearmint, which took ages for me to find, because it is the FA concentrate that I have for this flavour, not the Inawera, doh.
I tried one drop of that mix in approx 1ml of choc fudge mix, and not a lot happened, but at two drops, there was my hint of mint without any overpowering.
As I said, rough and ready, but that would translate as 1 drop of spearmint concentrate to 5ml of a choc fudge brownie mix, but tastes do vary of course.
(Of course, all drops not being equal, if I were to make up some myself, which I am likely to now, I would initially either put five drops of the 9/1 mix in 5ml, or 1 drop in 10ml, before fully committing myself)