Cut extrude with an offset

Hi to all, i need to make a simple offset cut on my curved surface, i usually do that in this way:

  1. Project a Split Line of your sketch onto the surface (Insert, Curve, Split Line… Projection)
  2. Offset (Copy) the Split Faces a depth of zero (Insert, Surface, Offset…)
  3. Cut-Thicken each open profile (Insert, Cut, Thicken…)

The problem that i have is that the offset copy is not perpendicular to the surface as you can see in figure, so the cut will be skew...

Can anyone help me?

SolidworksParts And Features