The empirical point about professional calibration

For users who have company or internal devices without calibration option

After watching the clip, notice this fact that you should consider one line lower to determine the target point, which means if you have obtained a scan from your visual metal detector with the method mentioned in this video of Poyeshgaran and you have seen a point on the third line in the visualizer since you have performed the first line without movement, thus your target is on the second line.