the amount contained in a dropper - https://www.collinsdictionary.com/dictionary/english/dropperful

A dropperful is the amount of liquid that fills the glass tube of the dropper top when the bulb on the dropper top is squeezed and released. The liquid may fill the glass tube only a small portion of the way, but that is considered a “dropperful”. - https://thymewisper.com/thymes-tinctures/what-is-a-dropperful/

there is too much room for misinterpretation of what might constitute a dropperful. - https://www.consumermedsafety.org/medication-safety-articles/item/102-what-is-a-qdropperfulq