Rhino doens’t have parametric sheetmetal, that’s a very specialized thing. Even if your CAD HAD it…well are YOU making these parts or are you having them made? 'Cause the way workflow works in Rhino is to just model what you want the result to be and then the parts supplier will model it up in Inventor or something with all their own k-factors(which can vary from machine to machine) and the right corner details and make you the parts–which they may do regardless of your CAD. If you want a general idea of what the flat sheet will be like for quoting, model the parts as a single thin polysurface, usually along the middle of the sheet thickness, which is easy to flatten.