For the moment forget about percentages, because not all TV/TT combinations are created equal and everyone has different loading habits, plus different TV suspensions react differently to loaded weights.
How a WDH works:
The main function of a WDH is to return the same weight that was removed from the TV's front axle do to the "effect"
of the TT's loaded tongue weight being placed on the TV's hitch ball (TV's front rises, rear squats). The importance of returning the same weight removed back to the TV's front suspension is to re-establish the steering integrity and TV handling characteristics.
Under most correct WDH adjustments, the TV's rear will still be handling some added weight so the TV's rear will still squat 1" to 2" (normal, but not as much squat without the WDH) even though the TV's front suspension has been returned to it's original condition (weight & height).
The WDH when engaged will distribute (remove) weight off the TV's rear axle then distribute the proper amount of weight back to the TV's front axle and a lessor amount of weight back to the TT axles. The weight being distributed in most cases is less than the loaded tongue weight value (not all TV/TT combinations are created equal). NOTE: The loaded tongue weight originally placed over the TV's hitch ball remains fairly constant, the WDH doesn't move weight off the TV's hitch ball.
If you look at the WDH ball mount where the spring bars are inserted into the ball mount sockets, as the spring bars are engaged there will be a for/aft force taking place within the sockets thus transferring force through the TV's receiver and the TT's A-frame.... this in turn translates into weight being distributed off the TV's rear axle. The tongue weight sitting "on top" of the ball mount remains constant.
Sometimes folks will say that about 20% of the tongue weight was distributed back to the TT's axle, but in reality the actual weight comes off the TV's rear axle. Depending on the TV/TT combination, this could be a 10% value.
The only way to confirm the weights being distributed is a visit to a CAT scale, which everybody should do. The CAT scale will confirm all weights and confirm the WDH is adjusted correctly.
Hope this sheds a little light on the WDH (a lot to digest). If I can clarify anything, please advise.
On Edit: When you have some time, walk yourself through the CAT scale weigh-in process and how given weights are compared, may be helpful: http://www.jaycoowners.com/showthrea...igh-Your-tt-tv