SOLUTION: One must press the use key [e by default] to lock on to the grid point desired.

Example: I walk up to my old explosive barrel again, and again see the 2x2 grid on its top. I pull out my tool gun to attach a hoverball, but instead of shooting, I press the use key [e]. It seems I was too close to the barrel, as I accidently pick up the barrel in my hands. I set the barrel down again by pressing my use key, and take a few steps back. I again view the 2x2 grid on the top of the barrel, and press my use key when I see a small red x appear in the center. The small x turns green, and I shoot -- absolute precision! The hoverball is directly in the center of the barrel's top.