From the other thread on this:
It's a simple balance equation. Weight of one thing times distance plus weight of another times distance divided by all up weight gives the location of the balance point. Convention has weights forward of the reference as positive, behind it negative.
Typically for aircraft the datum point is ahead of the nose, this keeps all sign convention simple.
If I were to move the elevator servo back on my FlatFlier AD-5W, the c.g. change would be like this...
Just where it comes out with finger balancing...