...may have been mentioned already. But if you click any waypoint, you can use it as a new starting point; just click a new spot for the next new waypoint. This will delete anything after it and leave everything before it intact.
Search patterns are one of the most useless features in the nav toolset.
It would've really been cool if they would let you modify those on-the-fly. Unfortunately, they are all preloaded at game startup. Imagine how cool it would've been if they just read the textfile during gameplay, you could change em at will.