Would you mind if I joined this discussion?
The more the merrier as with all threads ..
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.
This stuff is all the same whether we are talking a glider, C172, or B747. Once you get the head around the techniques it is all a bit of a doddle. Getting the head around it in the first place is the problem.
Think I'm just saying the same thing in different words?
Absolutely ... but why make something simple so complex ? Doesn't appear to achieve anything to a simple Simon like me ...
Explanation -
If we ignore any IU shift for, say, a trimsheet, the general formula becomes
IU = [(FS-D)*WT]/C
which, on rearrangement, becomes
FS = [(IU*C)/WT] + D
and, if you plug that expression for FS into the stock standard %MAC equation
%MAC = {[(FS-LEMAC)]/MAC}*100
you end up with
%MAC = {{[(IU*C)/WT] + D - LEMAC}/MAC} * 100
where D = reference arm .. which is more generally referred to as the trim datum.