Cutting a part after bending?

I have rarely needed to use sheet metal, so I am very much a novice at it, but I would like to know how add an extruded cut to part such that it is cut after bending, rather than part of the flat pattern. I have a roll formed shape, and want to miter the end of the piece at 45 degrees. If I do an extruded cut through all, it appears to interpret it as if I want the angle there before the part is formed (see pic).

Any advice on how to make this like a regular saw cut through the finished part?

Thanks