the goal is to keep your trailer level. with the trailer sitting level measure from the tongue to the ground and then measure from your receiver to the ground. subtract one from the other and that should give you your drop. just keep in mind tongue weight and sag of the back of the truck. if the back end sags a whole lot because of weight just use a little shallower of a drop. just try to keep the trailer level. this will give you full range of motion on the hitch. I have seen to often when people use too shallow of a drop and when cresting the top of steeper hills the tongue will pop off because the range of motion is limited.
I find that for a common trailer, I use a little more drop than most people, as the Pathfinder's hitch sits high into the bumper rather than below it. I use a 4.5" drop to level out my boat and it's also really close with my utility trailer too. I do have a few different tongues with different drops laying around though in case I need them.