- Intelligent: Int = hp (+5 for humans, +10 for mind flayers)
- Semi-Intelligent: Int = hp/2
- Unintelligent: Int = hp/3
Wisdom is also usually the same score. These scores are used for comparisons to judge the effects of simple ruses or concealment, mostly. For other uses of mental ability, the category is going to override the score: semi-intelligent creatures can figure out some tools, but can't make anything sophisticated, and unintelligent creatures can't learn languages.
This does mean that bigger monsters (more hit dice) will have higher Int scores, but since the categories determine behavior, not the scores, this shouldn't matter much.