According to the arm cavity round-trip estimate in O4a, RTL was estimated to be 72 ppm for Xarm and 78 ppm for Yarm, using measured PRG of 14.5 and finesse of 1300 for Xarm and 1420 for Yarm 1420 for Xarm and 1300 for Yarm [edited on Apr 9; thanks Miyoki-san for the comment] (see JGW-G2415589).
There is a mystery that this is not consistent with the measurement at POP and POS (klog #30823). From this measurement, it was 50(7) ppm for Xarm and 61(7) ppm for Yarm (taking average between consistent POP and POS measurements).
In the POP/POS measurement, it was assumed that BS has R:T=0.2:0.8 for p-pol. Changing this ratio to R:T=0.17:0.83 or 0.23:0.77 will result in the measurement from POP and POS inconsistent by ~10ppm. This means that POP and POS measurements are pretty robust and BS having R:T=0.2:0.8 for p-pol is valid within this range (and this is consistent with in-situ measurement in klog #29279, while measurement for s-pol is not really 0.5:0.5).
Inconsistency between estimate from PRG could be attributed to ITM birefringence. In the estimate from PRG, it is assumed that arm loss is calculated from RTL*arm gain, but there is also loss from birefringence. If loss from birefringence is 3% (which is rougly consistent with previous measurements in the presence of Lawrence effect; see klog #30823), 30ppm inconsistency can be explained. 3% also can come from mode mismatch, as single arm modematching is 97(1)% for Xarm and 94(1)% for Yarm (klog #33306). Mode mismatch can be created with inhomogeneous birefringence.