Thank you!
Thank you John Tullamarine, ft and everyone else for the excellent explanations.
I come here for this topic every now and then to refresh my knowledge. I also have the FAA Weight and Balance handbook, and it's pretty neat.
Still, it's good to know the name of the technique, transposition, for calculating the arm from the datum.
I need to know where do these calculations come from in order to fully understand them, I don't like just swallowing in formulas without knowing the meaning of them.
Again, thank you all folks!