How to optimally utilize the cargo space by predicting the unused space, pre-assigned to vendors

In the mid-term capacity planning process for air cargo, a cargo carrier reserves capacity up to six months in advance for its clients, who provide regular and frequent shipments over multiple flights. We model the underlying capacity allocation problem as a portfolio optimization problem to allocate cargo space on flights while minimizing the demand co-variance between allotments and spot market demand. 

Know on how to optimally utilize the cargo space by predicting the unused space, pre-assigned to vendors.