I would also bevel the edge to get the initial shape.
Img1: Create your base shape - remove all the extra edges to start with until the base shape is right.
Img2: Bevel the edge with the size and amount of segments you want.
Img3: Select either corner, press D to get into pivot edit, then align pivot with top most vertex and the right most vertex.
Img4: Scale the selection, (do it on front and back) to get the difference you want.
Afterwards you need the create the rest of the edges to support the shape and remove the Ngon from the front and back.
There are many ways to do it, but this would be my approach to it