Police Headquaters and Public Services Building

I am currently working on a Police Headquarters and Public Services Building. This project utilizes a parking deck adjacent to the building and reserving spaces for both police vehicles, staff vehicles; while public visitors will have access to paid parking not specifically reserved for the building.  How do we calculate a baseline to define the reduction? There is no real base line indicated on the chart. Also, how do you handle parking required by the police for vehicles that are used as tools of the trade?