I can't answer the question with numbers but I have a similar situation towing my 23" trailer with a 2007 Chevy Silverado.
I back in to my driveway that has a pretty steep slope and the street in front of the driveway is crowd with a low gutter at the start of the driveway. The only time I've scrapped anything was when I disconnected the WDH before backing in. Removing the WDG allow the front end of the trailer to dip and scrap the driveway. Since then I leave the WDH connected until I get up the driveway and haven't had the problem.
