both of my 5ers but will reply. The truck bed hitch heigth is adjustable, about 4-6 inches. The upper deck lower edge is the critical measurement. When sitting level, the bottom of the upper deck was about 55" above the ground. So your bed rail should height be 48" no more than 50". These figures are for a 3" lift so factor that in with a non lifted trailer.
When you unhitch and you pull your truck out it is good to record the height. Once you pull out from under the trailer you will usually usually lower the nose about 4" to get the trailer level again. If you raise the nose to the recorded height before hooking up it makes hitching up a little easier.
If I had not traded my truck I could measure the dent on my old tailgate and give you an exact figure.