Would you mind if I joined this discussion? I've done what QAYS is trying to do for several aircraft but only Boeing and McDonnell Douglas, never Airbus.
However the principle is the same.
You need certain basic information before you start (as has already been mentioned), most of it available in the manufacturer's manual.
LEMAC (leading edge of MAC)
Length of MAC
Reference arm
Constant (used to convert index into manageable figure)
When you have the operating empty weight and index, it becomes
relatively easy then to calculate %MAC.
%MAC = (Constant x (index/weight) + reference point-LEMAC)/length of MAC *100
Think I'm just saying the same thing in different words?