I will give my example. I have an F-150 4x4 ecoboost with the standard towing package. It has a tongue weight of 500 lbs. and a towing capacity of 5000 lbs.
I add the WD hitch and the towing capacity goes to 9800 lbs. Ford figures 10% on the hitch so my WD hitch weight also increases.
My Dutchmen trailer sticker specs show a dry wieght of 5003 lbs it has a tongue wieght of 486 lbs. It has a max total wieght of 7700 lbs so I can load 2700 of cargo. This would of course give me a tongue weight of 770 lbs.
Now in the real world I am at 1000 lbs or so of trailer cargo, so my trailer trailer tongue weight is about 600 lbs.
Everything is within spec and I have no problems with this combination. I just wish I could get 10 mph instead of 9 plus.
There is nothing mystical in these numbers. The only unusual thing I noticed is a 5000 lbs trailer with an 800 lb. hitch wieght.