All equation's I've seen for Millikan's oil drop experiment involve quantitatively knowing the Voltage beforehand. I don't see a way to calculate the Voltage using Coulomb's law without knowing the correct unit for coulombs, and I don't see how this could be done either using Ohm's Law, seeing as the unit Ampère requires knowledge of the quantitative coulombs per second.

Could you describe a logical chain from first principles of how Voltage is measured? Thanks