This page calculates the shape to cut out to form a cylinder of a certain length having a different diameter at each end, or a truncated cone. If you know what shape you want, in terms of the size of the circles at each end (diameter or circumference) and the height, this page will tell you the radii of two circles to draw and the angle of the section to cut out.
The inputs are as shown here:
You need to know the diameter or circumference of each circle, and the length of the side of the cone segment (not the free-standing height).
The result of the calculation will be the radii of two concentric circles and the angle of the wedge of the circle:
