In most cases 2"s ahead of axle center would be considered ideal. You should have posted your rig specs, truck, and trailer. I think you have a Ram 3500 dually long bed. With a standard bed, you would want to stay close to axle center, to allow more trailer to cab clearance. In your case, long bed, you should start out 2"s ahead of axle center, with the arms in the more forward position. With your heavy TH, you will likely have some weight added to your front axle, in the arm forward position. At some point, you need to weigh the rig, to know you are in the best arm position, for your rig. I am only guessing, but you will probably add about 200 lbs. to the front axle. You don't want to overload the front axle, when you have plenty of room on the rear.
Jerry