That 20 drops / ml is the default in eJuiceMeup. Accurate? Probably not!
For tiny amounts, I use a small syringe, sans needle and plunger. Cover the outlet with a finger tip, drip in the desired amount, then remove finger when over mixing vessel.
For tiny amounts, I use a small syringe, sans needle and plunger. Cover the outlet with a finger tip, drip in the desired amount, then remove finger when over mixing vessel.