Ok, so yesterday we established that we’d be looking at a d6 tally system with a baseline difficulty of 4. We want to expand on that so as to better handle a die pool of up to ~5d6. Given that, there are two different vectors of approach here. The first is the purely mechanical, while the second is conceptual, speaking to the role of numbers. They weave together, so let's five in from one direction and see what comes of it.
First, we'll start with numbers. Now, one great thing about d6 systems is that the numbers are pretty well known, and there are decades of games trying to come up with interesting fixes to smooth out the progression of the average roll, since that troublesome 0.5 makes life complicated. These decades are part of why, as I noted yesterday, 4 is such a magical number. And the good news is, there are a few other magical numbers.
The first candidate is 7. 7 is a great number for two reasons. First, it's the likeliest outcome of a 2d6 roll, and second, it's the first roll that's outside of the possible scope of a 1d6 roll. That latter is handy because it provides a fantastic model for something that may not be hugely difficult, but which requires training to be able to accomplish. For example, even if you know how to use a computer (1d6) you don't necessarily know how to write even a simple program. That requires specific knowledge and training - you're not just going to "get lucky" if you keep trying. That's exactly the kind of scenario where a difficulty of 7 is a handy tool.
After that, the next magic number depends on how you look at things. 13 has many of the same benefits of a 7, except that you need at least 3d6 to hope to hit it. The difference is that it's a bit less likely to succeed - 13 on a 3d6 is harder to hit than a 7 on 2d6. Now, this might suggest a compromise middle step of 10, since that's a midpoint on 3d6, and it's +3 from 7 and -3 from 13, and given that 7 is 4+3, and 4 is 1+3 (1 being guaranteed success), there's some numerical elegance in making the progression 4,7,10,13 (especially because it can be expressed as "base difficulty of 4, with quality of success increasing by one 'step' for every extra 3", something similar to what's done in a few other dice pool systems).
The problem is there's some conceptual roughness to it. I mean, yes, I could easily say:
4 - Mundane
7 - Difficult
10 - Complex
13 - Boggling
or something equally pithy and it would still be better than a lot of games (which set their baseline too high) but really it would be utter bullshit. Those terms are crazily subjective, and while I don't object to the GM interpreting situations, they provide the GM no practical guidelines for how those things are actually set, which would be irresponsible of me.
But if we drop the 10 we get something that's not quite as intuitive a progression, but is one that gives us a real, concrete basis for the progression: the numbers are such that if you are not at least operating at a certain skill level, you can't hit them. That means 4,7,13 (and 19, if we really feel it's necessary).
Now, what those things will mean are going to vary depending a lot upon the specifics of the skills, and when this ends up in a system, some of that is going to have to be offloaded, but the basic progression is pretty simple.
4 - Normal. The difficulty for day to tay tasks that might be difficult, but require only familiarity with the action being accomplished. For example: Disinfect a cut, perform the Heimlich maneuver.
7 - Expert. Difficulty for a task which cannot be accomplished without at least some proper training and experience. For example: Perform more advanced first aid (proper splints & bandages) or give CPR.
13 - Master. Difficulty for a task which requires intense, specialized training. Example: Perform Surgery, prescribe drugs.
19 - Past-master. Perform a hyper-specialized task. For Example: some sort of specialized medicine, like brain surgery or the like.
All of which is to say, the difficulty levels are built on a clear understanding of "Do Not Try This At Home". If this is something that anyone with a little familiarity could do with luck or hard work, then difficulty is 4.
Now, I should note that I view success as trickling down. If you hit a 7, you also implicitly hit a 4, which may suggest certain bonus or not, depending on the task. If you are doing neurosurgery (19) and roll a 17, then the failure is in the specialized part of the activity. In contrast, if you rolled a 12 (lower than needed for less complex surgery) then the problem was with the surgery as a whole. You get a lot of meaning trickling down through the tiers.
Also, doing this implicitly folds in duration of activity. Climbing a mountain is a task anyone could conceivably accomplish(4). Climbing a mountain in an afternoon probably requires training and experience(7). If you roll to climb the mountain and roll a 6, you still successfully climb the mountain (you beat a 4) but it's going to take you longer than an afternoon (since you didn't hit the 7).
Now, this is still just a starting point. We still haven't added in fiddly bits, and we haven't subjected it to the real test - a conflict system - but this seems like a solid start.