Another sideline point re datum position -
(a) the position of the datum is entirely arbitrary .. ie it doesn't matter where you choose to hang the end of your tape measure as the end result will be the same (different numbers but the same significance). Usually, the OEM will choose to place the fuse station datum somewhere out toward the nose of the aircraft, solely for convenience.
(b) however, once you choose a datum, then ALL calculations relating to each other must be with reference to that datum otherwise it all turns to worms.
(c) be wary when your aircraft uses trimsheets for loading systems. It is typical for the weight control person to use the OEM datum for the weighing stuff but then choose a different datum (usually around or at the aft limit of the CG envelope) to minimise trimsheet execution errors.
I had one occasion, many years ago, where a mate (who also was a weight control engineer) redid some configuration stuff on a cabin class twin, reissued the empty weight data (good boy) but then, without thinking what he was doing .. reissued the loading system (trimsheet) entry data using the OEM datum .. which made it incompatible with my trimsheet .. fortunately the chief pilot was the next to fly and, being a mate, gave me a call to sort out what was going on ...
Weight control is straight forward and really only requires good housekeeping .. but you do need to know what you are on about ..