You want 10-15% tongue weight for any WDH system.
Once connected...the term tongue weight...forget about it.
I tow with an SUV that is actually the very scenario you mention. WITHOUT my WDH attached I exceed GVR and RAWR. With the WD attached, I am within all capacities. My own calculations told me I would be over spec regardless of WDH. But the scale says otherwise.
My actual tongue weight is 960lbs. My WDH has 1400lb bars and transfers 520lbs back to the front axle and almost 200lbs to the trailer. Moving 500+ off the rear axle to the front axle brings me well within rear axle spec and 200lbs to the trailer brings me more than 100lbs under GVR. (That's fully loaded, packed as we would for a long trip.)
There is no specific percentage nor calculation you can easily use to accurately estimate any given WDH with any given tow vehicle. There are far to many variables.