Depends on the mini dropper you use as to what size drops they produce from their droppers. Reports are that most commonly sold ones do indeed have smaller dropper sizes, however the sample eye drops dropper I repurposed from my eye Dr. (which looks identical to the purchased ones) have the same sized drops as the original AM bottles.
I know this, because I put aprox 110 drops from the original AM bottle into the dropper to fill it, then got aprox 110 drops back out when I emptied the mini into the main bottle again. I recommend the same test to determine the proper drop ratio for any new bottles used.
Also, I've posted the question before about the practice of premixing, as it is a common practice as you say. The resulting comments from BPL were pretty unanimous that it's an acceptable practice. However, the only evidence anyone can produce, is that either "I haven't got sick", or "He/she does it, so it must be ok". Worthless anecdotes coming from mostly mtn stream drinkers IMO. Now if I were to hear these claims from a bunch of swamp-water drinking midwesterners, I'd be more convinced.
To the contrary, I've read multiple responses from McNett, and have received one of my own from them, stating how the "off-gassing" that occurs once the agents are active renders the solution much less effective, and basically becomes "flat" over an undetermined short period of time.
For me, it's not worth going through all the trouble of treating in the first place, if it's only going to be half as effective, just to save 5 minutes of wait time.