I´ve been trying to get a flat pattern of this sheetmetal part. I was able to to make two edge bend but no luck on flatten this part.
Info no.1: Part is created in Creo 5.0
Info no.2: Your "partial cone" is not cone.
Use Shaded curvature analysis to get gaussian curvature of the "partial cone" surface. You will see that its value is not equal to zero. The surface is not ruled surface, therefore you cannot create flat pattern.