The amount of load you want to transfer via the WDH is the only criterion for determining the capacity of the WDH.
The purpose of a WDH is to add back some or all of the load which was removed from the TV's front axle when the TT was attached with no WD applied.
Restoring load to the front axle which was removed by adding weight behind the rear axle is not a purpose of a WDH.
When weight is added behind the rear axle without a TT attached, load is removed from the front axle and it is not restored.
So, when weight is added behind the rear axle with a TT attached, it is not necessary for the WDH to compensate for that portion of the load removed from the front axle -- only for the portion which is removed by trailer-induced vertical load.
IOW, since you don't compensate for effects of behind-the-axle load when you're not towing --
you don't need to compensate for effects of behind-the-axle load when you are towing.
And, if I a major on-line hitch retailer told me that weight behind the rear axle becomes part of the bars ability to transfer weight and cargo behind the rear axle actually affected the tongue weight --- I would be seeking advice from another souce.
Ron
