public int requestedMipmapLevel { get; set; }

Normally, the mipmap streaming system selects which mipmap to load based on the position of all Cameras. Setting this field forces a specific mipmap to be loaded and overrides the mipmap streaming system. The mipmap level can still be modified by budgeting memory. Use the Cubemap.streamingMipmapsPriority property to increase the chance of a particular Texture mipmap level being selected by the mipmap streaming system.

Additional Resources: Cubemap.ClearRequestedMipmapLevel.