Iraq Body Count use to have a page that broke down who was killed and by whom. For some reason that page is now gone. But from memory I recall that 1/3 of the total civilians killed was attributed to the coalition and the other 2/3rds attributed to insurgents, terrorists, and violent crime.
So I usually take their total and multiply it by 1/3 for the number killed by the coalition.
The page is still there. It's the database link found in the middle of their home page.