Here is my setup when I tested for clearance.
I was at nearly 90 deg and had 4-5 inches to spare. I could have went to 90 without hitting. I'd have to be backing at an extreme up angle to hit and wouldn't want to be at 90 deg then anyway. Being at that angle really torques the trailer axles so you wouldn't want to do it unless you have to.
With my B&W set at up at the most rear point "I" don't need the slider. Your results might be different.