When you fold sheet metal, the material on the outside of the bend is stretched, while the material from the inside of the corner is displaced into the corner (metal being non-compressible). In order to account for this material displacement, this material is taken away by a line half-etched through, as EJFoeth already said. A width of the thickness of the sheet should be sufficient.
The folded angle then should measure on the outside the width as drawn plus the thickness of the sheet, e.g. 1.8 mm + 0.2 mm. Unfolded, the strip would be 4.0 mm wide with a 0.2 mm half-etched line in the middle.
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