I recently did a towing course and one of the trainers briefly discussed the WDH and it's setup.
From memory he said that when you have the van level (not attached to tow vehicle), measure the height of the hitch from the ground, then your tow ball should be at the same height (once the van is attached).
Don't measure the same height without the van attached as the tow vehicle will sag when the van is connected and it will be too low....so you need to compensate for the sag, basically.
Also, the WDH should help so that that front wheel arches are not much different to what they are when the van isn't loaded (ideally). There will likely be some bias towards the rear but if it's setup correctly the tow vehicle should be reasonably level (and whatever difference there is at the front arches should occur on the rear arches (so if the front goes up by 10mm, the rear's should lower by around 10mm when the van goes on). If it's wildly different the setup is not correct.
Hopefully I've explained that without too much confusion. Any experts out there that can ellaborate (or correct me), please feel free
Dave