I never measure using drops. I use mL or g to measure flavors. In particular with flavors I use mL since I always use syringes with those. Quite frankly, trying to measure by drops is unreliable. The dropper in dropper bottles come in all different sizes not to mention that the viscosity of pg, vg, and even flavorings can be radically different depending on temperature. If the goal is to create a consistent flavor using drops is unreliable unless we're talking about something that calls for such small amounts of a substance like 1-10 drops per 30mL.