Given the dimensions on the image it appears that the curvature is elliptical. It should be noted that Contour Base flanges only allow circular arcs and lines...
Here is the result I get when using arcs and lines...
and here it is with an elliptical sketch, it won't even recognize the sketch...
I even tried creating a Thin Extrude body...
and when attempting to convert to sheet metal only the Tab and first bend were created...
