Idiomdrottning’s homepage

Inverted DC

Sometimes you wanna know the DC to fail at something.

Subctract the original DC from 28 and there’s your new inverted DC.

For example, our house rule when you drop a torch is a DC 12 Wisdom save or the torchlight is snuffed out.

But what if you want the light to go out as you drop it? That’s a DC 16 wis save.

I’ll walk through the reasoning behind that number.

Of course, you can just do what Unknown Armies does and just be like “I hope you fail the roll” but for the sake of math curiosity I’d like to figure out another way to run those moments.

Step one, let’s just look at keeping your chance to fail the same but keeping your roll direction “you wanna roll high”.

Here are some example numbers. Let’s say the base DC, pre inverting it, is 12. If you have a +0 to the roll, then the inverted DC is 10. (45% vs 55%.) But if you have a +6 to the roll (just pulling a number out of my hat here), and you wanna keep the exact same chance to fail but just invert the direction, then the inverted DC should be 22. (75% vs 25%.) The general formula is to subtract the base DC from 22 but add twice the mod.

This formula gives the same probs as “trying to fail” does. The better you are, the harder it is to fail.

That raises the question: if I have a good stat but I’m actually trying to fail, shouldn’t more skill mean more control mean more chance of making it go the way you wanted?

Also, it goes against the entire blorb vibe to be tweaking DCs based in the character’s stats.

Luckily, there’s a shared solution to both those problems: Let’s assume a baseline mod of +3. Maybe it’s an off-stat at a low-tier proficiency, or a good stat without proficiency. This means that to invert the DC for deliberate failures, subtract it from 28.

Using this method has the unintended but totally awesome sideeffect that if your mod is really lower than +3, it’s harder to “deliberately fail” than to accidentally fail because you’re so bad you just don’t have the control, and if it’s exactly +3 it’s the same, and if it’s more than +3 it’s easier.

Obv different rule sets / xp curves will have a different “baseline number” than our +3.