What NK needs is to make clear that the cost of attacking it can be unsustainably high, while provoking the least possible response.
NK doesn't need to fire anywhere close to the US in order to do this. We're already watching them, and for the most part, they've already made it clear that attacking them might cost us a city or two.
Putting on a nuclear waterworks in front of Los Angeles would be psychotically risky, because the US would need to decide whether or not to return fire before the US target is known. "Vaguely near Los Angeles" might as well be Los Angeles because of measurement uncertainties, late-stage course corrections, and MIRVs.
As usual, the winning move is not to fire anything that might be construed as a nuke towards any of your enemies, unless you are prepared to die.
That doesn't seem smart at all. I think that a nuclear explosion on the ocean, right in front of a major city (NY, LA) but at safe distance, would be a better start. It provides the same amount of threat with no victims and no pressing need for blind retaliation.