Identify the exact line
Work on the visible boundary rather than repeatedly cutting the entire panel. Strong light and a clean viewing angle make it easier to isolate the line from natural shadows caused by head shape or density.
Use gradual lever changes
Begin with the setting that safely addresses the line, then make small lever adjustments as needed. A light cornering motion can target a narrow area without removing unnecessary weight above it.
Control pressure and movement
Heavy pressure and long strokes can push the transition upward. Use a deliberate flicking or scooping movement that exits the hair as it reaches the upper edge of the working area.
Reassess from a distance
Brush loose hair away and step back. If the mark changes as the head turns, it may be a shadow or density variation rather than a guideline. Continue only when you can identify what the tool needs to change.