Getting the DC current in dV/dI circuit

I have a simple circuit:

R1, R2 and R3 are actually one sample that has 4-terminals. The series resistor is to ensure current-driven mode. The voltmeter and Ammeter are actually Lock-In amplifiers synchronized with the AC source ($V_{AC}$). I use a frequency of 13 Hz. The DC source ($V_{DC}$) is a Keithley sourcemeter, while the addition of the AC and DC is done using a summing amplifier.

I am going to do a dV/dI measurement by sourcing a fixed AC bias (say 1-100 mV) and varying the DC bias (say from -1 to +1 V).

Q: how to get the DC current flowing in the circuit? Can I say: $$I_{DC}=I_{AC} times frac{V_{DC}}{V_{AC}}$$
If not, what is the way to do it?