Success with Flow!!!

Using Half of the cone as a Solid, and another silver copy of the outer half face of the half solid as base surface, the solid will flow correctly to a planar unwrap outer surface.
However, rather than using Planar on the unwrap, which produces rectangular isolines, use sweep of one of the unwrapped slantseams, to sweep the arcs of the unwrap frustum.
This produces the proper isolines.

I do see some little artifacts on the smaller arc...?
And no confirmatory measurements were made.

- Brian

I'll have to try the full frustum solid...