How do you constrain the circumference of a parabola in the sheet metal tool??

I am using a piece of 4X8 sheet metal to bend into a mirrorin the shape of a parabola. This is easy to do in real life but notso easy to do in SolidWorks.

How do I create a sheet metal object of this mirror as a parabolaand constrain it's circumference to make sure it accuratelyrepresents what the 4X8 sheet of flat metal can do?

SolidworksSheet Metal