Cone (geometry)

I'm restoring some 10' wooden colums that are tapered. I've decided to replace the bottom 10" with a plastic material. Not that it matters to the problem but the diameter of the top of the truncated cone is 14.96" and the diameter of the base is 15.28".

I now believe the correct term for what I seek is developable surface. So, if I cut out the bottom 10" of the cone then slice the cone along it's "Locus?" then lay it flat (2D) (which I can't because it's wood) the top of the resultant shape would obviously have a slight arc on both the top and bottom edge and the sides would be angled.

How do I calculate what the diameter of the circles that describe the arcs? I need to know this so I can cut the plastic with the correct arcs prior to forming them.

Thanks
Bill