Drop hitch, 8" or 10" drop ?

Looking to get a tandem axle car trailer, unable to measure hitch height and hoping that someone can help out in deciding if I need a 8" or 10" drop hitch for my 2005 F350 SRW extended cab.

Looking at the adjustable aluminum one with locks from Diversi-Tech which is rated at 1500# tongue and 10,000# trailer weight while weighing only 20# itself. The aluminum is supposedly 30% stronger than steel. This model has locks and is reversible for storage.

[image]http://www.trucktoysautomotive.com/images/Adjustable%20Page%20copy.jpg[/image]