I know we've got some fairly smart people around here, so I thought I'd throw this at y'all and see what sticks.

I have a fantasy RPG in the works, though the genre is irrelevant. It currently is pretty much a d20 clone. I'm working to change that. Dropping the classes in favor of skills. Skills. They currently are percentile based. There are six ranks of skill, Unskilled providing a -25% bane, Novice at 0%, Apprentice with +5% bonus, Journeyman with 10%, Master at 20% and Grandmaster at 30%. Target numbers are Trivial (25%), Easy (40%), Moderate (55%), Difficult (70%), Hard (85%), Challenging (100%), and Impossible (120%). So as you can see, you currently need to be a master or gmaster in a skill to hit an impossible task.

Now each skill has a stat associated to it. The stats run from 0 to 10. This will change to -5 to +5.

I'm looking at changing the core mechanic to a dice pool system. You get +1 die (1d10) for each rank in the skill plus one die per stat. But Erik you say, what about negative stats? Those provide bane dice. So if you are attempting a skill you are Journeyman in (3d10) and you have a stat of -2 for that skill, you'd roll 5 dice; 3 bonus dice and 2 bane dice. The bane dice trigger on the absolute value of the stat, or in this case a 2 or lower. Each bane die success negates a regular die success. Currently I'm thinking a Trivial task has a TN of 5, Easy is 8, Moderate is 10, Difficult is 10 with 2 successes, Hard is 10 with 4 successes, Challenging is 10 with 6 successes, and Impossible is 10 with 8 successes.

My issue stems from the fact that I am trying to keep relative percentages the same, but I'm not sure how to calculate percentages with a die pool.