.006 is safe, but not optimal. You want to go smaller for better accuracy. Ideally, you should gap it at .006, then back it in close enough to the cylinder that it doesn't scrape -- take into account carbon build-up, a possibly out-of-dimension cylinder, and heat expansion.
My W-12 has a wonky cylinder, so I gap it, screw it in further, then hold it up to the light while I turn the cylinder to make sure there is no contact.
EDIT: The idea is not to go over .006. I have an S&W that needs to go back to the factory because the cyl. moves and allows .008+ gap sometimes. It is highly inaccurate, and it spits yellow-hot sparks out the side! Ouch!
