i have a couple real A4,s to copy so its easier...i mark an outline with a pen. slight back as you can see, i use a micrometer and make sure the measurements are good, then with a drawfile that the same width, i slowly make a cut, at the same angle that the first sweep cut is done, then with a medium sized ball cutter on my moto tool, i start the rounded cut, some masking tape around the edge is good...you will slip.
keep in mind that this and my cut is made for a real A4 bolt or one of my replica sniper bolts..most sporterized bent bolts wont clear the notch.