You want to match the proportions of the shade and the base. Put
the base on a large piece of paper and draw the outline. Thus, you receive
a two-dimensional picture. Now outline the form of the shade and measure
the diameters of the rings as well as the height.
Ro = radius
of top rim
H = height
Uo = circumference of top ring
Uu = circumference of bottom ring
Ru = radius of bottom rim
Formula to calculate
the shape of a conical shade
Hint # 2:
The bigger the difference
is between the two rings, the smaller the radii become and the easier
it is to draw the model.