If you are using the differential voltage method across the external series resistor, take the voltage waveform observed, and divide by the resistance of the resistor. This will result in a current. There is a chance that the resistor impedance is not exactly resistive across all the frequencies we are interested in, but the parasitic capacitances and inductances are generally low, as long as you aren’t using a wire wound resistor. Most designs nowadays that I observe are using thin or thick film resistors for the external series resistors, which are definitely fine for this measurement and application.
