pitman arm pro comp question

hey guys I was just wondering how much space should be between the back of the pitman arm and the steering box. Basically I have a pro comp 6" lift with a vibration and had read online about people pitman nut backing off. mine is on there but there is about a 1/2" gap between the steering box and the seat of the pitman arm. Also I can see thread on the steering box shaft still with the pitman still on.should it seat higher up?. I know its tapered but I dont know how far it should seat up on the shaft.