I think I am right in the following. If not, I'm sure Ron or somebody else will be along to correct me. :)
Roughly speaking you have added around 700lbs to the payload.
That 300lbs that was transferred back to the truck front axle is weight that was leveraged off of it by placing the tongue on the ball, not weight added from the tongue. The weight on the axles before drawing up the WD bars would be around 1300lbs - 1000 from the tongue and 300 from the front axle of the truck.
Then about 300lbs of the 1000lbs is transferred back to the trailer axles leaving about 700lbs of the 1000lbs left on the truck to be subtracted from the trucks payload.
Barney