I usually play crucible and use snipers in my special slot.
I use kontrol freeks, a long one on my aiming stick.
Also I play on a 24" monitor
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It's what ever feels comfortable but from my pov i think 7 or 9 would be best and it also depends console because the sticks are different on each console like i like playing 7 on xbox one but on 360 i like playing 9