disclaimer: I am not an expert.
My experience probably differs from everyone else's, but here's what works for me.
I test 5ml batches in 6ml bottles so I've got room to add stuff if something's way off. For me, doing smaller sample sizes just makes for a larger margin of error and with a 5ml batch I can do the math in my head.
I may catch a little flak for this but I never, ever count drops. I always work in percentages so that my results are both scalable and repeatable.
If I'm doing something new I generally do a 0mg test batch unless I'm working with flavors with which I'm pretty familiar. I generally start out with total flavoring at 10% - this is where the extra space in a 6ml bottle comes in handy. If there's a dominant flavor I usually start that at 5% and the other flavors make up the other 5%. I test with a debridged 306 atty so I can just put one drop right on the coil and
vape it until it's gone in just a handful of hits. Flavoring tends to rest on the top of the mix so I generally shake well then pitch the first two or
three drops out of the dropper to make sure I don't have any flavoring stuck in the dropper itself.
I'm beginning to learn that I need to use about 3/4 as much flavoring as I think the batch needs as I've found a few that were great when fresh and a bit overpowering when they steeped for a week or so.
I mentioned this in another
thread but I account for every drop of flavoring. I take good notes and transfer the notes to a spreadsheet so I don't lose them. I think the paperwork is as important as the mix.