I need to create an arched accent above a garage door. The distance between the bottom of the header and the soffit is 16". The accent needs to be 8" in the center and 4" on each end. Is there a way to create this without finding the center of the circle because it would be below ground level?

Are you saying you need to cut a piece of material in to this shape?

Yes. The problem is that I am doing the accent out of cement and need to install the plasterstops on the top and bottom instead of just installing a solid piece of material.