It sagged 3.5 inches without the WDH and you adjusted that to a sag of 1 inch correct? I think that you moved to much weight off of the hitch. Also, an overly adjusted WDH can lead to some really bizarre handling issues. Adjust the WDH so that the truck sits about level and than adjust the hitch so that the trailer sits level. If that solves your sway than start adjusting the WDH and removing load from the hitch until the sway comes back and then back off. Also, look to see if the sway bar is adjustable. If it is than start tightening it up but I would do my first suggestion first because using the sway bar to compensate for a bad WDH set up will probably lead to a little ditch digging.
If you are looking for a rude and crude method of determining how much weight is on the hitch than all you do is stand on the ball of the hitch and have someone measure how far the truck sagged. Bumper or wheel well does not matter but bumper works better. If you weight 200 pounds and the truck sags 1 inch than count 200 pounds per inch of sag so 3.5 inches is 700 pounds. Its crude but its close enough to give you an idea of where you stand.