That run is not that bad. I would take a step back from that helmet for a week and let it cure. If the run still bothers you after its had a chance to cure, just wet sand it with 800 or 1000 grit wet or dry sand paper. After its lightly sanded hit sanded area with some novus plastic polish. If it brings out the paint then I would try hitting the whole helmet with novus. You can also try a nice car wax.
I really dont like Krylon Fusion at all for high gloss painting. Its a really easy paint to use, but it never really shows a nice ABS like gloss. Its not a bad paint for Sandtroopers or roughed up clones though.
I have discovered that on most paints if you lay a coat down and let it sit for 5 minutes and then re-coat, wait 5, re-coat wait 5, it loooks pretty darn good. Of course you will want to make sure that yoou have prepped the surface well and made sure its smooth, dust free and oil free. Paint in an area that is not windy, cold. Never paint at night with the porch light on..... Bugs love fresh white illuminated paint...I dunno why.
Always follow the cans directions, and dont buy cheap paint unless you want a cheap finish.
Thats about all I can think of. I look forward to seeing your helmet finished!