Scott Wilbur
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Challenge Rating system for Fate

In the last game I ran, I had some trouble setting the difficulty of conflicts that the PCs would get into.  I think some of the issue was the large weapon values you can get in the Dresden Files RPG, but I also wished that the rulebooks had more detailed advice on how to scale the opposition to the PCs.

Therefore, I made a Monte-Carlo simulator for Fate conflicts, ran it over a wide range of conflicts, and distilled the results to a system that's easy for GMs to use.  As it turns out, you can fit the Monte-Carlo results with a very simple system:

Calculate a character's challenge rating: (Attack Skill) + (Defense Skill) + (Stress Boxes).  Obviously, Og the Barbarian probably has a higher CR in a physical fight than in a social conflict.  If you're sending one opponent at a group of PCs, the group has a CR equal to (average CR of the PCs) + (Number of PCs) + 1.

Decide what type of conflict you want:
A: The PCs will win, and they will look awesome doing so.
B: The PCs will win, but they might need to spend some resources.
C: The PCs will probably win, but there is definite risk involved.
D: The PCs have approximately 50-50 odds.
Obviously, the PCs can achieve a better result than this by having good stunts, creating advantages with their best skills, and working together well, so a "D" conflict is reasonable in the right circumstances.

Decide on the CR of the opposition:
A: PC's CR - 4
B: PC's CR - 3
C: PC's CR - 2
D: PC's CR - 1

Build the opposition to match the CR you just calculated:
An opposing NPC has a CR calculated the same way as a PC. (Subtract 1 if they're unwilling to take any consequences, or subtract 0.5 if they're unwilling to take a moderate consequence.)
An opposing nameless mob has CR = 2*(Skill) + (Number in mob). (I use stress more like hit points for my nameless mobs, like Spirit of the Century.  If you use full stress boxes for them, they'll have a slightly higher CR than this.)

For example, a PC with Fight +4 and 3 stress boxes (CR = 4+4+3 = 11) can handle a mob of 5 Fair NPCs (2+2+5=9) or an NPC with Fight +3 and 3 stress (3+3+3=9) in a tough, but winnable fight.  A group of 5 such characters (11+5+1=17) can handle a giant with Fight +6 and 4 stress boxes (6+6+4=16) only if they're well prepared and work together.

I was planning to use this system to rebalance Ryan M. Danks' excellent Adventure Fractal system (http://ryanmdanks.com/?p=499), but then I discovered that he had already adjusted the adventure skills to exactly the numbers I would have chosen based on this system. (Apex, Apex-1, Apex-1, Apex-2)  The only change I plan to make to the Adventure Fractal is to set the stress to 1+(Number of players) instead of 3+(players/2).