I don't really mix by drops, too inaccurate, I've used syringes with blunt dispensing needles in the past (1, 3, 10 and 30ml), they worked great but were a PITA to clean and maintain. I currently use digital scales, quick and easy.
I have several different sizes of glass droppers, for the largest (like the ones used in 30 - 60ml glass bottles), 25 drops = 1ml, for the smallest (fits WL's glass 8ml flavoring bottles), it takes 45 drops to dispense 1ml. Different sizes and types of droppers dispense different amounts per drop. The viscosity (thickness), of the fluid being dispensed also effects drop size and therefore changes the number of drops required to = 1ml. The viscosity of PG and VG changes dramatically with temperature so the drop per ml ratio is somewhat temperature dependent. Best way to determine the drops per ml ratio of a specific dropper and fluid is to count how many drops it takes to dispense 1 ml of the fluid being measured. I used a 1ml syringe barrel for this purpose.