5th wheel hitch position in the bed

So a little forward adds some stability? OK Chucking is the front to back banging around tendency of the trailer? Have not heard this term yet.. sorry I'm learning. My truck is an 02. MMM...my install of the rail kit to my understanding only mentions that the centerline of the Kingpins' base has to be 2 inches forward of center in my bed as to clear the bed supports (sills), not to where the hitch Kingpin head should be or could be... a B & W has many ways to adjust the head position (up, down ,forward, aft)...this is why I ask these questions. IS it just so as your trailer can clear the cab and the head position doesn't matter? As I read all of the install instructions of bed rails for different trucks, it seams to me it has to do with clearances of the base rails to sills of the bed not the Kingpin head position on the base of the hitch. It is all over the map as to measurements of the base. So now I am confused.