It's a shame that knowing average monster hitpoints is generally metagaming and there is no ranger option or similar to show you this.
It would be cool to follow a fireball. If you know the enemy you're fighting has about 32 hitpoints for example, such as the thug, and a band of them got hit by a fireball for 30 damage, sleep is a perfect spell. But getting this combo off in game always feels a little metagamey in a way that just makes it ineffective.