SizeDimension of the shipment depending on business rules

I have to adress a transportation problem quite similar to the TransportOfDisabledPeople case.

Where there is seat for the DisabledPeople and seats for medical staff.
The rule i want to implement is the following :
For a the transportation of ‘normal illness’ people.
The transportation of 1 people needs 2 medical staff.
The transportation 2 peoples needs 3 medical staff (so one less)
The transportation 3 peoples needs 4 medical staff (so two less)

The objective of the problem is to do the maximum of transportation (Shipment) given a number of medical staff and vehicules (small and big).

I don’t know how to implement that in jsprit. Because the addSizeDimension method of the Shipment Builder is now part of the optimisation probmem.
Do you have any ideas of how to implement that kind of case ?

Thanks in advance for your help