Well, start with these three assumptions:
- Ability scores are relative, not absolute, and rated as 0 = half normal, 10 = average, 20 = double normal, The extremes (0 and 20) are never actually used, because those equal a score of 10 on a different scale.
- A giant has the strength of two men (because it does 2 dice damage, instead of 1 die.) Thus, Strength/10 roughly equates to dice of damage.
- 4 points of damage equals one die of damage (because the average damage for 1d6 is 3.5, which rounds to 4.) So, +2 damage = half a die of damage, and +3 damage is the same as an extra die of damage -1 point.
Since the scores from 9 to 12 are reserved for "normal" Strength, Strength 20 is only 8 ability points above normal. That's +1 damage per 2 ability points. That leads to the following damage scale for humans:
- Strength 9-12: 1 die of damage
- Strength 13-14: 1+1 damage
- Strength 15-16: 1+2 damage (ogre damage)
- Strength 17-19: 2-1 damage
The damage adjustment for low scores on any scale would be a flat -1 to any score below 9.