I've had much more success recently in designing both encounters and monsters by
throwing out the CR system entirely and focusing my encounter work on three metrics:
- The sum total of the party's hit points versus the average Damage Per Round of the enemy combatants
- The average Damage Per Round of the entire party versus the sum total of the enemy combatant's hit points
- A combat length of three rounds
- Choosing a target difficulty
There are two steps
1) Ensure that the sum total of the enemy combatant's hit points are sufficient to survive the DPR of your party for three arounds. I feel that combats longer than three rounds risk getting boring, but if you like your fights to be drag-out slog, adjust accordingly.
2) Determine the DPR your monsters will need, decide on what percentage of the party's total hit points you want to bump off, assuming a four person party.
- 25%. At this level, the only time a party member would drop is with a serious tactical miscalculation resulting in one party member taking the brunt of entirety of the damage. This would be an easy/ medium encounter.
- 50%. At 50% hit point reduction, there is a sharper chance that a player will drop or die. Even in an ideal scenario, most party member can expect to be reduced to half hit points, with maybe one or two being unscathed if the others take most of the damage. This is usually sufficient to cause the party to "sweat" but only a grave error or bad rolls is going to swing this the other way. This is appropriate for a medium to hard encounter.
- 75%. At 75% hit point damage inflicted, the party members still standing are gravely wounded, and they are doing so only barely or because two or more have dropped. Be aware that any situation in which more than one party member drops in a four person party can quickly escalate to a TPK. This is a very hard or deadly encounter.
NOTE: If you're using multiple creatures, check the DPR of your party against their individual hit points to check for how many the party can kill each round. The result is that your monsters have a higher DPR in round one, average in round two, and low in round three due to casualties.
Thus, If we had, for example, a party of four characters with a total hit point pool of 100 (about what you could expect for third of fourth level characters, or six second level characters) and an average DPR of 40/ round total, for a hard encounter to select 120 hit points of monsters, who have a sum total DPR of about 15. Under your example, choosing two or three Carrion Crawlers would be right on target for a medium/ hard encounter, which seems appropriate based on your description.