Using drops as a measurement is very inexact! 40 drops equaled a ml for me. But, now that I know that, I can adjust accordingly.
I generally interpret recipes as 1 part this, 3 parts that, 2 parts the other thing and so on, so you can use that for drops, percentages, and ml measurements after you do the math.
Some flavors are much stronger than others. I always start light on the flavors - you can always add more, but you can't subtract it, you have to add more PG &/or VG &/or
eliquid. By that point I've lost track of all of the measurements and would have trouble re-creating something I liked!
I'm still experimenting with flavors so only make small batches of about 2ml and just count the drops after I figure out how many drops of each ingredient will equal about 80 drops.