sean The "bit" that causes vignetting! Seriously, just shoot an image wide open, marking the hood location relative to the image orientation, and where you see vignetting is where you need to shave the hood. How much is a different matter, depending on what you decide is the desired result.
OldPhotos "If everyone possesses some measure of this intangible quality called creativity, photography is unprecedented as an outlet for its expression." - Ansel Adams