Where the stock runningboard leaves 2-3" of fender exposed, the body kit exends all the way to the fender opening and is still 1-1 1/2" wider there. It establishes a line that is picked up by the airdam and carried across the car.
The other strategy shows in the MG EX 181 aerohead posted. The large-radiused rocker combines with the tight-fitted spats.
Which is best? :shrug: The MG is likely better in a cross-wind but the design isn't really compatible with doors. Consider a rocker that leans out 30° vs one that leans in 30° (similar to your body kit). Intuitively, the 'fillet' shape would offer less interference drag between the ground and the car's side.