Or, you can say them to someone who is not already in full rage, but has already left the arena of calm. They're working up a stew.

The trick is, the phrase must be immediately preceded by or followed by the reason why they're getring off track.

It also helps to have a relationship or familiarity with the person or situation.

I tell my teenager to calm down a lot. According to conventional wisdom this is all wrong. But she's never complained, she's never more enraged, and she does in fact calm down.

The problem with "calm down" is that most people use it dismissively or as a weapon. It's a phrase that can leave someone hanging without those riders.

I would say don't ever use that phrase alone.