Travel costs where multiple destinations: how to appropriately allocate costs where equal apportionment is not possible

It may not be possible to apportion matters equally where more than one destination is involved.  For example, you might be travelling from Glasgow to Edinburgh for one client, then going on to see another client in Dunfermline and then returning to Glasgow.  You should consider the timings of meetings and the distance to travel before working out the most economic way.  In this example, the second part of the table below shows how to attribute the cost to each case correctly, although not equally, as this is not possible for multiple destinations.

Overall mileage and time Mileage Time
Glasgow to Edinburgh 48 00:59
Edinburgh to Dunfermline 20 00:40
Dunfermline to Glasgow 50 00:56
TOTAL 118 02:35
     
Client 1 Mileage Time
Glasgow to Edinburgh 48 00:59
Dunfermline to Glasgow÷2 25 00:28
Total 73 01:27
Client 2 Mileage Time
Edinburgh to Dunfermline 20 00:40
Dunfermline to Glasgow÷2 25 00:28
Total 45 01:08
Overall mileage and time 118 02:35

(Outward journey allocated to appropriate case, return journey apportioned equally.)

In this section