How to Modify Base Attack Bonus

[Biggus]

Inspire Greatness – gives 2HD (up to +9HD if you invest in the ability), which also gives BAB (but discuss with your DM)

I've never heard this interpretation before. How do you decide what type of BAB those extra HD give (1/1, 3/4 or 1/2)? If those HD were meant to give you BAB wouldn't it specify what type like it does for how many HPs they give you?

Edit:

Investiture of the Pit Fiend – set BAB to character level (Heroes of Horror II)

Just noticed there's a typo here, it's in Fiendish Codex II.