Drag the three points on the plan to where the centre front of the sink, hob and fridge will be. The triangle should be compact but not cramped, and no leg should cross a tall cabinet or the main walkway.
In a single-wall kitchen the three points sit in a line, so the triangle can't apply. The tool then checks the run length instead.
Sink ↔ hob–
Hob ↔ fridge–
Fridge ↔ sink–
Total–