It is nice to know that your combination tows nicely. That is huge. When you are dealing with a WD set up it is pretty important to get three passes over the scales. Follow the link that lawrosa included.
I would like to know how your setup looks. Is everything level? Adding 1000lbs to the back while removing almost 400 lbs from the front does not sound right to me. You should always try to replace as much weight to the steer axle as possible.
You asked "how much weight can I add?" Add where? Are you planning on loading up the bed of the truck or are you going to put extra stuff in the trailer? It will make a difference.
What kind of WD hitch do you have? You can do an estimate by measuring the height of your truck above the drive wheels. When you connect the trailer adjust the hitch to return this height as close to the original as possible. Mine is something like 1.5 inches. Load the truck and trailer with what you will typically be using and get it weighed using the worksheet. There should be a weigh station close by.