Using the edge finder:

When G54 X = 8, G54 Y = 0.158 on one edge of the fixture
                G54 Y = 5.356 on the other side of the fixture

      as the Starrett 827A has a 0.200inch tip,
      hence, center of the fixture  = ((0.158 + 0.1) + (5.356 - 0.1))/2
                                    = (0.258 + 5.256)/2
                                    = 5.514/2
                                    = 2.757 inches

When G54 Y = 2.757, G54 X = 11.22 on one edge of the fixture
                    G54 X =  1.00 on the other side of the fixture

      as the Starrett 827A has a 0.200inch tip,
      hence, center of the fixture  = ((11.22 - 0.1) + (1 + 0.1))/2
                                    = (11.12 + 1.1)/2
                                    = 12.22/2
                                    =  6.11 inches

When G54 X = 6.11, G54 Y = 0.162 on one edge of the fixture
                   G54 Y = 5.360 on the other side of the fixture

      as the Starrett 827A has a 0.200inch tip,
      hence, center of the fixture  = ((0.162 + 0.1) + (5.360 - 0.1))/2
                                    = (0.262 + 5.260)/2
                                    = 5.522/2
                                    = 2.761 inches

Hence, center of the fixture is at  G54 X = 6.11 inches  = G55 X of 5
                                    G54 Y = 2.761 inches = G55 Y of 2.5