Yes, 20 percent is used as an average. I think more loaded FWs, will be over the average, than under.
The dry weight is often figured on the low side. If you are figuring to add 1,500 lbs to the trailer, I would figure for a loaded total of 13,500 x .20= 2,700 lb pin weight, plus another 200 lbs for the hitch.
I think your axle/tires could handle 3,000 more lbs, but you would be at the edge of RAWR, especially depending on what else is going in the truck. Another thought the 6.2 will be at max around 12,000 lbs, unless you have 4.30 gears.
Jerry