With your current set up, you will not be able to separate the difference between two records, especially if there are more than 2 'stops'.
For example, if you have 5 stops. Using the Min stop time, and Max stop time. Because that will give you the first stop of the route, and last stop of the route, not the difference since the last.
However, you have 2 other advanced options.
1- If you are adding stops sequentially, you can modify the button to write the 'previous stop time' into a field on the new stop
or (2) you can create a circular (self) relationship, and the next stop will basically be a child to the previous stop.
These options are dependent on the workflow, how the stops are added and sequenced.
