[Design] The Difficulty with Difficulty Numbers

So, for the last while I've been stressing over where to start with designing an RPG - with the setting, or with the mechanical structure? It turns out, the answer is fairly wishy-washy: both. I'm reminded of all those people who cannot settle on a firm answer, saying "well it depends..." as if it were some sort of religious mantra, and all those times I've wanted to kick them in the junk. And now, I'm saying it.

Wishy-washy. Gah, there's an awful taste in my mouth.

So, you work on both at the same time. You work on the setting for a little bit, and then you establish the basic mechanics, and then you use those basic rules to expand upon your setting, which then informs you where you need to work on your mechanics, and so on and so on, a nerdy sort of positive feedback.

I now have a basic map of the setting figured out - a ruined city haunted by giant worms and ravenous cults; a floating "barge" city that functions as a metropolis for trade; a military state based off a fuelless aircraft carrier; a confused semi-religious state calling itself "Little India"; and a commune of former hippies that has declared psychic powers illegal and prosecutes psychics (the twist? Everyone in the community is a psychic, and just hides it from his neighbours).

Sounds cool, right? Well, that's not what I'm going to talk about here. Instead, I want to talk about Difficulty Numbers. Because, hey, those are fun too.

I've got the basics of my mechanical system figured out - a percentage-based system with a boolean "Success or Failure" mechanic, similar to the systems used in Call of Cthulu as well as Warhammer Fantasy Roleplay - both games I'd love to play, but never have. The system is also a cousin to the old thieving skills from D&D and the skill system from RIFTs - both games I've played, and I actually enjoyed the one that didn't, you know, suck.

Essentially - you have a base percentage of success, you modify it for difficulty, and if you roll under it on d100, you succeed.

Here's the problem: what is the success rate for a task of average difficulty? Some games will say that a character should succeed on an average task 75% of the time, while others will say 50%. Since I'm using a system where difficulty numbers are modifiers to a character's base chance of success, should an average difficulty have a modifier of +0%? Or something else?

This ties into another question: where should a character's base skill rating start at? 50%? 60%? Higher? Lower? There's a lot that goes into this, and I could go on for a while. But I do that too much already, so I'll cut to the gist of things.

I decided that a character of average skill should succeed on an average task 50% of the time, provided he does not prepare for the task (he doesn't aim, he doesn't get help from a companion, etc.) It's an arbitrary starting point, but it makes sense in my head to sort of level the "middle of the road" for everything at zero. If anything, it makes the math easier.

Difficulties modify your base chance in 20% increments, though nothing will stop GMs from using the smaller 10% modifiers. An Average difficulty task has no modifier to your attribute, while an easy task gives you a +20% bonus; likewise, a hard task will stick you with a -20% penalty. So, an average character with a bit of training in the skill at hand (base skill 50%) will succeed on an easy task 70% of the time, and a difficult task 30% of the time - seems good to start with.

In fact, I think the game seems hunky-dorey. I wish I could post up my math sheets, but there's a lot of big math errors on it, as well as Help me Satan, my dark lord and master written in goat's blood right near some poorly-conceived bell curves.

Oh, and a Hello Kitty sticker.

Because who doesn't love Hello Kitty?