In order to quantitatively evaluate how much Finesse asymmetry can be reached given the control accuracy of Etalon obtained in O3 (total RMS of 6mK), the simulated Finesse behaviour vs Etalong, for the two cavities, has been simulated with Finesse (this study has been done in the framework of the discussion of having or not the Etalon for Adv+ phase II).

The optimal working point corresponds to the Best case scenario where the two mirror temperature is chosen to match the maximum of the West arm cavity finesse and the corresponding value for the North, to minimize the asymmetry (see Figure 1).
This should correspond to the best sensitivity wrt etalon, for this reason the Etalon working point chosen in O3 was the one that maximized the BNS range.

In that configuration the finesse asymmetry obtained, considering the control accuracy of 6mK, is of ~0.05%.

In the worst case scenario, Figure 2, where the slope of the Finesse curves is maximum the asymmetry achievable is about 0.15%.

This computation is done in the assumption that the wp has been chosen correctly (the one that minimizes the finesse asymmetry).