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)?