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