Attached is update to ConeUnwrap script.
The .js is considerably shorter.
The .htm is a little larger, with a bit more detail in the prompts.
The math is the same.
The orientation of the cone/frustum in space does not matter. Lengths are used, which are positive numbers.
The Bottom and Top of the frustum can be picked in either order. If Top Length is greater than Bottom Length, they are swapped.
Substitute Bottom, SlantLine, and Top circles can be used. A substitute Slant Line can be placed anywhere the SlantSeam would rotate around the center axis of the cone/frustum, which could help avoid holes. (A rewrap cone would have a relocated slantseam.)
A portion of a cone/frustum made by trimming with two SlantLines, can also be unwrapped to a Net, using matching Bottom arcs and Top (for frustum) arcs.
The Base and Top circles/arcs do need to be planar, and the cone/frustum "right circular."
Thanks again to Michael for his help and hints, and borrowed .js and .htm code :-)

With subsequent Flow unwrap of addition surface curves/edges/holes, the user will be responsible for verifying orientation of everything.

- Brian

ps If all goes well, I'm going to do ObliqueConeUnwrap next, which will include elliptical or circular bases, or portions thereof, provided they are planar.
The equation will require numerical integration, which is already available in another script, and interpcurve.
The Oblique script should also work with cone-like objects with "radial" "ruffles" or wrinkles along the SlantLines (I think.)
The object will have to be developable, and have an apex point, although subsequent Flow should handle frustums.
(We'll have to see how the bottom edge turns out.)