I usually put the chrongraph out as far as the leads will allow. Which is about 15'. That way the escaping gas from the muzzle does not trip the first screen and show a lower velocity. I would think 10" would be more than enough as the escaping muzzle gas slows very rapidly, I would bet after 5' muzzle gas has slowed below bullet speed. But if you always go to end of the leads you are measureing at the same point from the muzzle each time. Not 10' one day, 12 feet the next...ect, just makes it more consistant.