= Loss measurement from cavity reflectivity =

The power of reflection when locked: P_lock = P0 (1 - M * 4T_ITM/T_tot² * (T_loss+T_ETM)),
The power of reflection when ETM is misaligned: P_mis = P0 (1-T_ITM),
where M is the ratio of the power of carrier TEM00 light that matches to the cavity mode.

From the cavity scan measurement, M ~ 0.95 (with a few parcent error). And from the reflectivity measurement, P_lock/P_mis = 0.6440(6).
Note that we normalized the power at REFLAIR_A by IMC_CAV_TRANS_OUT, so that the power fluctuation at input optics can be negligible.
So the roundtrip loss T_loss can be calculated to be
T_loss = 410 +/-10 ppm.
The error comes from rough estimation of M, which will be updated by the detailed analysis of the cavity scan.

Discussion - clipping loss?
According to this document by Kiwamu (senario1), miscentering on a test mass by 4-5 cm will result in the roundtrip loss of hundreds of  ppm.
Looking at the picture on ETMX, there might be miscentering by 4-5 cm, which can explain high loss and the fact that the losses we got yesterday and today did not match.
We have to redo the measurement after we confirm that the beam spot on each mirror is well centered.