As a byproduct of the previous project, but perhaps useful on its own, here is a table of all the specific percentages you can get if you add advantage and disadvantage to a d20 roll.
| Percent chance | Die expression |
|---|---|
| 0.25% | 1/20 with advantage |
| 1% | 2/20 with advantage |
| 2.25% | 3/20 with advantage |
| 4% | 4/20 with advantage |
| 5% | 1/20 |
| 6.25% | 5/20 with advantage |
| 9% | 6/20 with advantage |
| 9.75% | 1/20 with disadvantage |
| 10% | 2/20 |
| 12.25% | 7/20 with advantage |
| 15% | 3/20 |
| 16% | 8/20 with advantage |
| 19% | 2/20 with disadvantage |
| 20% | 4/20 |
| 20.25% | 9/20 with advantage |
| 25% | 5/20 |
| 27.75% | 3/20 with disadvantage |
| 30% | 6/20 |
| 30.25% | 11/20 with advantage |
| 35% | 7/20 |
| 36% | 4/20 with disadvantage |
| 40% | 8/20 |
| 42.25% | 13/20 with advantage |
| 43.75% | 5/20 with disadvantage |
| 45% | 9/20 |
| 49% | 14/20 with advantage |
| 50% | 10/20 |
| 51% | 6/20 with disadvantage |
| 55% | 11/20 |
| 56.25% | 15/20 with advantage |
| 57.75% | 7/20 with disadvantage |
| 60% | 12/20 |
| 64% | 8/20 with disadvantage |
| 65% | 13/20 |
| 69.75% | 9/20 with disadvantage |
| 70% | 14/20 |
| 72.25% | 17/20 with advantage |
| 75% | 15/20 |
| 79.75% | 11/20 with disadvantage |
| 80% | 16/20 |
| 81% | 18/20 with advantage |
| 84% | 12/20 with disadvantage |
| 85% | 17/20 |
| 87.75% | 13/20 with disadvantage |
| 90% | 18/20 |
| 90.25% | 19/20 with advantage |
| 91% | 14/20 with disadvantage |
| 93.75% | 15/20 with disadvantage |
| 95% | 19/20 |
| 96% | 16/20 with disadvantage |
| 97.75% | 17/20 with disadvantage |
| 99% | 18/20 with disadvantage |
| 99.75% | 19/20 with disadvantage |
| 100% | 20/20 |
Not saying use this always (it’s cumbersome af that the target number jumps up and down) but can sometimes (rarely, admittedly) be useful when you’re looking to port over a specific probability from some other system and want to leverage their playtesting. (If you have reason to suspect that a particular rule is not well playtested, then of course you don’t need to approximate their probability either. Just use what would make sense in your game instead.)