This comment may be a litle late to help Carles Quero but I was having a big problem trying to put the pivot to the Unmasked areas trying to emulate Kent’s sphere cutting process in Blender version 4.3. I am now thinking that putting the pivot at the unmasked areas when you are mirroring the mask on the X axis will just result in the pivot remaining in the X-axis center of the object. Anyway, I think a workaround for 4.3 may be to use the Line Project command, which cuts the sphere but maintains the geometry.
Good luck out there………
