Can you just subtract the challenge rating of friendly NPCs?

Question

The party have bitten off more than they can chew and the DM wants to help them out to avoid a likely TPK by adding a friendly NPC to an encounter. Can they get a good sense of how difficult the encounter now is by simply subtracting that CR of that NPC from the overall encounter total?

A specific example: 4 level 3 characters facing an Oni (CR7). Would adding a Martial Arts Adept (CR3) reduce the encounter to about CR4, a hard but winnable fight?

I'm aware that there will be limits to this approach, 8 martial arts adepts will not stop an ancient red dragon killing a party of level 1 adventurers but does it work as a rule of thumb (if you stick within the same tier of play for example)?

Answer 1

No, you add the new creature to the allies side of the equation

You wouldn't subtract the CR rating from the enemy - that's still static. What's changed is the party fighting that CR.

You can utilize this question to help determine, roughly, the level of the creature you're adding and then utilize that additional creature and level for your encounter calculations.

The DMG covers this in Chapter 3 under Creating Combat Encounters, or you can utilize online tools like the Kobold Fight Club or Dhmholley's Calculator.

What those tools show:

Originally, the encounter rating would have been Deadly by a fair amount.

By adding the Martial Adept (approximate 10th level) to the party, it changes the rating to Hard (although at the upper limit for that.)