Actually, with a minim being just about 17 to the ml, a drop is just barely on the small side. A dropper tip drop of water (other liquids vary due to viscosity and surface tension differences) runs between 20 and 30 drops to the ml, though this also varies somewhat by the size of the tip the drop is falling from.
That's a pretty stiff solution, with something over 5 grams of salts in 12+ ml of water. I'd guess the exact strength isn't critical, though, and suggest you might want to just use 3 grams of each salt in 15 ml water (divided for mixing, though it's not clear to me why you'd need to do so) and call it good. I'd also suggest you'll need to multiply the batch a bit if you need to blacken anything bigger than a Waterhouse stop.