An easy process :)
Take the edge of the cone
Copy somewhere - Oriente / Line-line/none on the XY plane
Ask détails of the base cone circular length : copy to the ClippBoard
Arc Circle Center on the extremity segment
Copy the clippBoard inside box length of the arc circle
Close to the edges
Plane
Surface Cone = 1260.781055 (don't forget to kill the base surface!!!
Surface simulated "unwraped" = 1260.781248 not so bad!

Will be the same for an horizontal truncated cone - just 2 arcs circles
the trick is to ask the details! :)



The asked detail!