Although you have used the term "Sta" which appears to imply station numbers, the figures you quote are actually distances on the MRJT1 in the CAP 696.
You need to use two equations, Firstly to find the c of g position you need to use:
C of G positiuon = Total moment / total mass.
But first you need to find the total moment and total mass. You can do this by adding the cargo to the initial condition as shown below
Item Mass Moment Arm Moment = mass x Arm
Aircraft 41000 Kg +663 inches +27183000 Kg inches
Cargo 2500 Kg + 360 inches +900000 Kg inches
TOTALS 43500 Kg 28083000 Kg inches
Now using C of G = total maoment / total mass gives
C of g = +28083000 Kg inches / 43500 Kg = +645.59 inches
So the new C of G is 645.59 inches aft of datum
Now if you want to know the C of g positiona as a %MAC, you now need the second equation:
%MAC = 100% x (C of G - MAC Leading Edge) / MAC length
The MAC LE is at +625.6 and the MAC TE is at 760.1
So the MAC length is 760.1 - 625.6 = 134.5
Putting these figures into the equation:
%MAC = 100% x (C of G - MAC Leading Edge) / MAC length
Gives %MAC = 100% x (645.59 - 625.6) / 134.5 =14.86%