D&D 5E Switching 5e ability checks to a "fixed index"


(He, Him)
Taking the game text on PHB 174, and DMG 237 and 242, 5e offers a nuanced range of ability check results. Unfortunately for implementation at the table, they use a "sliding" index. For example, given DC 17 the outcomes are indexed by roll as follows
  • 18+ = Success
  • 15+ = Success with complication
  • 13+ = Failure
  • Lower = Botch
That "slides" with DC, for example given DC 21
  • 22+ = Success
  • 19+ = Success with complication
  • 17+ = Failure
  • Lower = Botch
A simple rule change that leaves probabilities unchanged is to translate DCs into modifiers, so that the group can use a "fixed" index, like this (the target number does not change with the DC)

Index (roll + modifiers)*Ability Check result
8+Success with complication

The DCs are "flipped" to modifiers, i.e. Very Easy +5, Easy +0, Moderate –5, Hard –10, Very Hard –15, Nearly Impossible –20. Once it's presented like this, I think one can see opportunities to improve the index without losing any nuance.

My 5th level Rogue with STR 12 attempts to climb a sheer surface with a long drop below. DM has classed the wall Hard. I have expertise with Athletics. My modifiers +1 +3 +3 –10 = –3. My roll (after modifiers) is 8 = Success with complication.

*These numbers might look a bit odd, but they're exactly those implied by DMG 242.
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(He, Him)
I've been running a 5th edition sandbox campaign for a few sessions now using a fixed index. I use the following table... but currently only for ability checks. It's going well, so I'm considering using a fixed index for combat results by making AC–10 into a negative modifer (similar to DCs). Example, studded leather gives a modifier on attacker's roll of –2. With a shield, that would be –4.

Index (roll + modifiers)Ability Check resultAttack Roll result
10+ nat. 20Success with increased impactChange a miss into a hit, or a hit into a critical hit
5+Success with complication?
LowerFailure with consequencesMiss
Lower nat. 1BotchChange a hit into a miss, or ?

The ability check column uses the full set of results given in 5e core rules, but there is no equivalent full set for combat. I'm considering using the following where there are "?"s in the table.
  • 5+ = Deal your damage modifier to your target, and they deal theirs to you (if in range)
  • nat. 1 = Fumble (a simple table of outcomes like dropped weapon / arcane focus)
Note that due to a personal dislike of every hit being a critical when hitting only on 20, I've modified nat. 20 and nat. 1 to offer a choice... sometimes forced (e.g. if you missed, you can't use your nat. 20 to change your hit into a critical hit.) I have in mind that perhaps there's an opportunity here to expand and do more with crits (at either end of the scale).
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I'm sorry to hear about your cat. We had several in our family.
Thank you for your thoughts. He may not be dying, but he doesn’t look good to me. We lost his housemate a earlier in the summer. So maybe I am just being pessimistic.

Regardless, I will let my DM know about this thread as he might be interested too.

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