Hi bz, a fillet like you're describing is not a developable surface, which basically means it's not physically possible to form it out of a flat piece of stock and then only bend it into shape along a straight edge.
Any surface that has compound curvature in it rather than being straight in one direction cannot be formed from a flat piece without having lateral stretching taking place at the same time.
That's part of why the whole area of surface unrolling for physical construction is a fairly complex topic.
Your 3D surface has to have the shape of something similar to a cone or a piece of a cone in order for it to unroll to a flat pattern.
- Michael
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