creating a contour flange has an inconsistent UI compared to the extrude command in the model. You can create a symmetric contour flange extrusion, but the value you enter is half the value you want the total flange extrusion to be. In the model environment, we can can choose overall or half.
In the image below the extrude command on the left is from the model workspace and the one on the right is from sheetmetal.
