Drops out of the same bottle type (or dropper) are not always the same size - sometimes they vary substantially (see example below).
When using concentrates or ultra-concentrates, drop size matters FAR more than if using a non-concentrated flavor (see example below).
Example:
16 bottles of VZ's Gourmet ultra-concentrated flavor. After mixing 16 different flavors at 3 different concentrations (being assured the drops were consistent by 'veteran users' in a thread in the Flavor Reviews forum), we found the results to be extremely inconsistent (stupid us, for not measuring as we knew was necessary for accuracy).
We then counted the drops per ml from each of the 16 identical bottles (squeezed by the same person, at the same temperature, held at the same angle). Range? A whopping 32 - 48 drops per ml. We repeated the test twice - results were similar, but not exactly-precisely the same.
We had mixed the test bottles using the 'standard' from the thread - 1 drop per 2 ml (and since we made three bottles of each, we also varied the amount of base in two of the bottles).
1 drop/2ml of 48 drops per ml is 1.05% flavor; 1 drop/2ml of 32 drops per ml is 1.57% flavor = a difference/range of .52%...a difference/range of HALF (of the commonly reported 'sweet spot' of 1% for those particular ultra-concentrates)!
So, if we apply that same difference/range to a flavor that has a sweet spot of 15%, the difference/range would be 11.25% - 18.75%!
The more concentrated a flavor is, the more essential it is to use adequate measuring tools (assuming one desires to create mixes & recipes that can be duplicated consistently & accurately), or be able to calibrate drop size very precisely. The difference in drop size might not matter all that much on a flavor needing 10-15% of flavor (it might just change the finished juice by a partial percentage) - but with concentrates & ultra-concentrates, even the slightest variance in drop size yields an enormous difference in the finished juice.
Of course, mix however you want (that's what DIYing is all about)! But please realize that one can be screwing newbies 'up & over' by generally advising that 'drops are accurate'/best for concentrates & ultra-concentrates...especially without explaining just how vast a difference 'drop size' can make with the concentrates.
Fwiw...our test size bottle is typically 10ml; after we've completed a recipe/formula, we never make less than 60mls at a time, and more typically make 120mls of each recipe.
***For those scientifically calculating drop size - note that due to the chemicals in each flavor being different (which may substantially alter viscosity), there's no way to ensure consistency in drop size across various flavors in a flavor line - even if the bottles, tips & base liquids are identical. The viscosity can also change substantially due to ambient room temperature.