You can change whether the final systems of flows always fill the width of frames or only do so above a certain fullness threshold in each layout independently. By default in Dorico Pro, the final systems of flows only justify to the full width of the frame when they are more than half full.
Procedure
Press Ctrl/Cmd-Shift-L to open Layout Options.
In the Layouts list, select the layouts in which you want to change the justification of the final systems in flows.
By default, the layout currently open in the music area is selected when you open the dialog. You can select other layouts by using the selection options in the action bar, clicking and dragging across multiple layouts, Shift-clicking adjacent layouts, and Ctrl/Cmd-clicking individual layouts.
In the category list, click Note Spacing.
Change the justification of final systems in flows in one of the following ways:
If you always want the final systems in flows to justify fully, deactivate Only justify final system in flow when more than [n]% full.
If you want to change the minimum fullness of final systems before they justify, change the value for Only justify final system in flow when more than [n]% full.
Click Apply, then Close.
Result
The automatic justification of the final systems in flows in the selected layouts is changed.
Tip
You can also change the width of individual systems independently of your default settings by changing their start/end positions.