The calculated CG and stab trim will be as accurate as the data which goes into the calculation. For a well designed trimsheet, reasonably accurate load information, and a modicum of care in executing the trimsheet, the error will be insignificant in the overall scheme of things.
Problem with ad hoc trim settings is that the certification requires a variety of stick force measures to be within acceptable limits ... the flight test program validates the AFM guidance .... the ad hoc do-it-on-the-fly stab setting MAY cause the flight test program work to be invalidated.
If there is a residual error in the stab setting, so be it. That's what pilots have muscles for ...
I know which approach I would prefer to argue in court after the accident ......