Storage Capacity : Use the service capacity
Use GLUE2StorageServiceCapacity
It seems that for computing the storage space of a SE, VAPOR taks into account the attributes GLUE2StorageShareCapacity(TotalSize, FreeSize, UsedSize). The storage areas can overlap each other, and in many cases this brings to an overestimation of the storage capacity. So the most proper attributes to consider would the GLUE2StorageServiceCapacity(TotalSize, FreeSize, UsedSize) ones.