Hitch Height
The general rule of thumb is the top of the ball should be 16 to 18 inches above the ground. This not as easy as it sounds, you also have to allow for the load you plan to carry in the bed of your truck and the tongue weight of the trailer. Also trailer tongue heights vary conciderably, it's best to measure your trailer to be sure. It's always better off to have the ball set slightly higher than level rather than lower. You can also get an ajustable drop stinger, the one that I have used goes from a 7. 5" drop to a 6. 5" lift and cost about $65 cdn. , with one of these the ball height can be adjusted to suit any trailer that you may encounter. I can email you a photo of one that I have on hand that I made to fit a 2. 5" reciever.
Neil