Mounted boss fin direction based on vector

Hello,

I may be wrong but it seems the fin direction of mounting boss is supposed to be based on edge or face.

I am currently testing a case where I don't see how I could orient mounting boss since the surface I am working doesn't offer edge or surface in the direction of the fin.

I have created a construction line going in direction of the supposed fine direction but it seems I can not select it in the property manager of the mounting boss.

Any reason for this ?

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