I found a few cone calculators online that will give you the details of the flat pattern to use that will roll into the desired cone shape.  I used rosin paper for the material, so I can cheaply make adjustments to the shape before committing to the real shade.

I setup a big trammel out of a strip of plywood, having a pencil hole at the two required radii.

It's looking like a lamp now.  I will make any necessary tweaks to the shape once the hoops I've ordered arrive and I can get their actual dimensions.