Barney,
All Bens answer says, it gvwr plus trailer wt needs to equal gcwr. If the truck is at gvwr, I can still subtract it from gcwr, and have the amount I can tow. Now if the trailer will tow say with a wagon setup, no hitch wt, I could tow that amount. But if one assumes the trailer wt needs to include a given amount of hw above and beyond the formula Ben is using, it will not work. You need to use 1.1 up to 1.25 times the difference, along with having free payload to handle the hitch wt for the tow rig to handle the trailer.
marty