Almost always when we talk about locating an airplane's CG, we're really only talking about locating it on the pitch axis. Front to back location is what we're talking about.

There are actually three axis that have a CG location. The location on the UP-and-Down axis (a new axis to lots of guys) is the one that makes our usual CG locating such a problem for low wing airplanes. When the CG is higher than the wing when we're balancing it, it makes the balancing act much much harder to do right.

When you turn the low wing airplane upside down, it suddenly becomes a more stable situation, AND how far above or below the supports suddenly matters a lot less.