Your in the range, I used 10% because I think I estimated your unloaded tongue weight up. I have another way of looking at things, many may not agree but my feeling is that when you are rolling down the road, the truck/trailer combo needs to react with road conditions and provide a good ride, in other words not too stiff or not too soft of a ride. My feeling is that if you have a pickup truck, it likely has a stiff suspension (as compared to SUV or car)and 800 lb bars (in your case) will give a little more spring in the ride to compensate for your stiff suspension. If you have an SUV, the suspension is softer and therefore you need stiffer bars to "tighten" up the ride. It's hard to explain, hopefully you understand my logic.
I don't quite follow you on the 4" lower thing. The sticky on setting up WDH is a little confusing if you are not interested in the mechanics. An easy way to set up is:
1) Measure distance from ground to top of tongue when on level ground or look for the number on the brochure.
2) Set hitch head on the shank so that the top of the ball is about 1" to 1-1/2" higher than the height measurement from step 1.
3) Measure distance from center of hub of front wheel straight up to the wheel well.
4) Decide how many chain links you want under tension on the bars so that you get the ground clearance you want.
5) Start with the hitch head tilted back toward the trailer and then 1 notch at a time using trial and error tilt the head up toward the truck until your front wheel well height returns to its unhitched height or close to it but not past it (Check your trucks manual for the recommended return - some are unhitched, some are 1/2", some are 1/2 the rise, etc.)
6) Once you get the tilt set, check to make sure your trailer is parallel to the ground it is on or slightly nose low. If nose high, drop the hitch head on the shank 1 notch and then reset the tilt on the head, if necessary (you may have to adjust back or forth 1 notch on the tilt.
7) Don't worry about the rear end drop of the truck. It is assumed to drop and that is why you start with the hitch head set about 1" to 1 1/2" higher than the actual height.
On edit: I'm sorry, I should have put this post in the other thread :)