Most planes recommended cg is slightly nose heavy to improve the chances of a successful maiden flight. Once you've flown the plane, you can adjust the CG within a range to achieve the flying style you prefer.

The plane feeling like a "floater" as you move the CG back is related to the tendency of the plane to drop the nose. A plane that is nose heavy will drop the nose at some point as the horizontal stabilizer loses its force to hold it up. With a neutral CG, the elevator doesn't have to apply much force to hold the nose in position, so the tendency to drop the nose is reduced, giving it that "floater" feel. As the plane becomes more tail heavy, other forces come into play and the plane becomes less stable, so don't overdo it.

For most aerobatic planes with symmetrical wings, the neutral CG tends to be right along the thickest part of the airfoil, which generally corresponds with the wing tube or main spar.

As for "tank empty", balancing with the tank half full is pointless. Always balance empty as this is the most rearward CG, and the most unstable condition for planes with the tank ahead of the CG.

Brad