I keep it at the max, which is 14 I believe. It takes a while to get used to but once you do it's worth it. Especially since in hardcore whoever gets that first hitmarker usually wins the gunfight. Always have to be faster than your opponent!
I keep mine anywhere between 8 and 11 depending on the game mode. any higher than 11 and I can hit anything and anything lower than 8 and it just seems like I don't turn quick enough