Ability Bonus
24-26 +5
21-23 +4
18-20 +3
15-17 +2
12-14 +1
09-11 +0
06-08 −1
03-05 −2
00-02 −3
(Lower than 00 is autofail)
Bonus = Score/3 −3 (round down)
Use only one ability for an attack using a d20. However, make damage normally use two abilities. For example, Heavy weapons add both Strength and Constitution to damage. Light (finesse) weapons add both Dexterity and Intelligence to damage. This helps flavor different fighting styles, and helps each ability to be more useful.
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It is also possible to make each ability score improvement meaningful, by ’staggering’ the bonus among save/skill, damage, and attack.
18 +3 to a saving throw or a skill check that uses this ability
17 +2 to an attack that uses this ability
16 +2 to damage that uses this ability
15 +2 to a saving throw or a skill check that uses this ability
14 +1 to an attack that uses this ability
13 +1 to damage that uses this ability
12 +1 to a saving throw or a skill check that uses this ability
11 +0
And so on.