The unaccounted IR drop is the resistance, caused by solution between the reference electrode tip (or Luggin capillary tip) and the working electrode, times the "current". But, in a 3-electrode setup, current between the reference electrode and the working electrode is negligible (as opposed to the current between the counter electrode and the working electrode, which is sensed/reported by the potentiostat).

So, which current should be multiplied by the resistance to calculate the uncompensated IR? Current at the working electrode/counter electrode? Or, the negligible current experienced by the reference electrode? According to my understanding, it should be the current experienced by the reference electrode, since it's the reference electrode that is approximating the potential. However, according to some literature, it's the current at the working electrode/counter electrode.