got a new truck and trying to work out ball height. the sticker on my trailer says 18" coupler height required? is this 18 from floor to bottom of coupler? says plus/minus 1.5" so not super critical? new truck is a 4wd and old truck was a 2 wd, so much taller bumper height. was using a 4inch drop upside down and boat towed fine? thks