You really should weight it properly..
Well read here on how to weigh on a single scale which you said you did..
http://www.etrailer.com/faq-how-to-determine-trailer-tongue-weight.aspxBut why didnt you weigh it with the WD bars??
Also you can weigh the trucks axles one at a time.
Anyway you need to go buy the axle rating that you never stated.. If you set the bars up right you should get some of that 840lbs off and on the trailer.
You can set the bars up to move as much weight you can as long as you resore from axle to stock ride weight, but never more.
Its really very simple.
In addition the the link above weights all you need is front truck axle ( truck only) rear axle ( truck only) And front axle with trailer and WD activated.. Then post your axle rating on the trucks specs..
I myself as I said are the same as you with weight being close to max. I wanted to know exact #'s because if I cause an accident and hurt someone I can at least know I was not over weight.
Just saying is all and only trying to help...