I took PR3 AR reflectivity and calculated PRG (PR3 is flipped and so AR surface is inside PRC).
As shown in attached figure, which shows AR specification of the LaserOptik mirror (PR3 is this mirror), AR reflectivity of PR3 is ~0.5 %. Since resonant light in PRC goes through AR surface of PR3 4 times per round trip, round trip loss due to this is ~2 %. Then I got
PRG = 7.8.