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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.)