Over time you become programmed to expect that adding a relationship between two topics with an arrow head at the destination implies a finish to start dependency. It did for several years but now there is an extra step.
This picture (click to see the full size version) shows how you define the dependency type. The roll up calculator ignores the dependencies until you do this.
You can select all relationships and define as Finish to Start in one action. Home Ribbon > Select > Select Special and then select the dependency type in the Task Management pane.
Now I can plan that event in minutes!