How do I calculate a monster's damage output per round for breath weapons?

Question

When creating a monster, on DMG p. 274 I am instructed to "Read down the Damage/Round column... until you find your monster's damage output per round."

So how do I figure that? Is it just the damage output of the creature's attack(s)? What if an attack could hit multiple targets, like the breath weapon of a dragon?

On p.277, under Step 11. Damage, we get close to defining this:

It doesn't matter how this damage is apportioned or distributed; for example, a monster might deal the damage every round with a single attack, or the damage could be split among multiple attacks against one ore more foes.

Okay, but how do I figure the damage for a breath weapon? It's going to be potentially very large variance depending on whether I think it's hitting one character or several.

Answer 1

After the section detailing the steps involved in monster creation, there is a table titled "Monster Features" that explains how to calculate these sorts of things. (DMG, pp. 280–1) Step 11 really ought to direct the reader to this table but doesn't, for whatever reason.

For effective breath weapon damage output, it instructs the DM to assume it hits two creatures and that both fail their saves.