I've decided to use Betty Boop as our Did Not Finish example.

To DNF (or DQ) a shooter, you can mark the check box on any of his stages.  I'm just using stage 5.

 

 

If you're in Shooter data entry mode, the stage at which he was flagged as DNF will appear in red on the summary for that shooter:

 

 

 

With either data entry mode, DNF shooters will be highlighted in red on the Select Shooter Scores to Enter / Modify screen.