Comparing the ghost beam spots in Fig 5 and 6, the distance from the main beam looks like 13.5mm and 10mm toward -X direction, respectevely. Their positions were 6x25 - 25 = 125 mm apart (toward +Y), so the angle from the main beam should be

(13.5-10)/125 = 0.028 rad. In the same manner for the pitch discussion, this would be 0.0028 rad at the entrance of the BRT.

Generally speaking, the displace angle of a ghost beam, which is created by a wedged back surface with a wedge angle of w and go through a medium with the index of refractance n, is 2nw. Substituting Sapphire's n=1.76 for 532nm, and considering the 2nw = 0.0028, the corresponding w is 0.0008 rad or 0.0456 deg.

According to Hirose-san (thanks a lot for your quick reply!), the ETMY now suspended has 0.05 deg wedge. So I believe the number is consistent for considering the measurement errors of the beam spot positions.