or any other low wing plane. Manual states to balance upside down and at it's c.g (100mm) it balances. If I turn it back right side up and balance from under the wing at the same measurement, it is tail heavy. Should I get the same result from both?
Thanks.

In theory you should.

But in practice, you often have a problem with where you place the supports. When they're placed on an angled surface, they often let the airplane tilt when it reacts to the angle. The top of the airfoil is tapering in the opposite direction than the bottom of the airfoil everywhere except at the point of maximum thickness. So if you put the supports anywhere on the win other than the point of maximum airfoil thickness. those supports are going to be trying to hold to an angled surface.

What usually happens when you try to balance a low wing airplane from underneath is that you first have a problem with the underneath's angles. Then the CG is going to be way above where the supports support the structure. Try balancing a pencil in your hand, point down, and you see the problem. The lower the wing is on the low wing, the harder it is to get it to balance steadily.

When you support any item like you do when trying to balance it, if the item's CG is below the support points, the item is positively stable. The CG will try to tilt the item with the CG moving toward the supports. It's stabilizing much like adding noseweight stabilizes the pith axis of an airplane in flight.

When you support an item with the CG above the support points, the item is negatively stable. The CG tries to tilt farther away from the supports.

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.