You should always figure your rates all miles. The truck doesn't run the DH miles on air. Rather you get paid money for DH miles or not add them in to get a true per mile rate for the load. If not you are cheating your costs and profit margin !!!
Might be a little left on a sprinter for you but even at break even its not costing you out of pocket and everyone wants to drive one. Just check the classified ads !!