Anyone seen the "signed" copy of the weight & balance which the Captain signed. The Captain would then have taken a copy for himself and a few copies would be left behind.
My point is, there are a few other copies of the cargo documents and weight and balance somewhere, which were left behind which would have a lot of info on it to start with.
These forms and cargo manifests would have signitures on them saying as to who prepared and loaded the pallets, and of course weighed them.
This of course would be a starting point. Considering the given weight, performance would be calculated, which the crew would've done.
From here on there may have been a big difference between what was believed and signed for, versus actual.