In the airline world it really is very simple.
The figures are all in the manuals. We actually have a laminated sheet in the cockpit with the weights and IU's.
It gives a Basic weight (with crew) with its index
A choice of 3 different catering weights with each index
Add the two together and you get a DOI
In addition there are published IU's for baggage/passengers and take off fuel.
Add it all together and you get a take off IU and hence a trim figure.