There is a trailer weight calculator floating around the Web but I don't have the link. Maybe someone will post it.
Basically, your target maximum weight is calculated by subtracting any weight of cargo and passengers from your payload and then using the remainder for the weight of a WDH and the tongue weight of the trailer.
If your payload is 1212, you subtract the weight of any passengers besides the driver and the cargo. Lets say your passenger weight is 200 (Mom and a kid). You now have 1012 left. Firewood, the camp chairs, the water toys or fishing gear in the bed all add up,lets say another 100 lbs. You're now down to 912. You buy a WDH and it's total weight with bars and all is 100 lb., 812 final tongue weight maximum. Divide that number by .13 and you will have the average gross weight target of the trailer at 6246. You may wish to have some leeway with that and aim lower. You may wish to ignore the payload ratings of your truck and pull more weight.