You need to understand the meanings of the various terms.
The MAC is the Mean Aerodynamic Chord Length.
The statement that
Quote:
the MAC is given as 134.5 inches

means the distance from the MAC leading edge to the MAC trailing edge is 134.5 inches.
15% of 134.5 inches is 20.175 inches.
So if the CG is at 15% MAC it is 20.175 inches aft of the MAC leading edge.
If we now add the MAC leading edge poistion of 625.6 inches aft of datum we get a CG position of 20.175 + 625.6 = 645.775 inches aft of datum.