Jim
The trouble with some of the stated numbers is that they are not based upon a fully outfitted rig. For example the tow capacity is based upon the truck GCWR minus the weight of a truck with no options, a 150# driver and a half a tank of fuel. Your truck in use will weigh significantly more than what they used. This is why you have to work up the weight of your truck when you are using it to tow your trailer. This is the weight to use to when adding the pin weight to see if you exceed the truck GVWR. I recommend you use 20% of the trailer GVWR. The trailer GVWR is usually on a plate mounted outside near the pin on the left side of the trailer.
I listened to a sales person who told me we would be unlikely to ever load our trailer to it's GVWR. When I weighed it the last time we were only 200# under the GVWR. By measurement our pin weight is 18% of the GVWR and this overloads our truck GVWR by about 300#. (This number varies as we load the trailer but not by more than 100#. ) The GVWR of my trailer is 10,640#, truck GVWR 8,800# and GCWR 18,000#. If I had realized that I would be routinely over the truck GVWR I would have bought a 3,500. Use only the truck and trailer GVWRs, truck GCWR, and actual measured loaded truck weight and ignore the other numbers the sales people give you.