The guy that bought my old Lance has a 15 Superduty 4 door and we had to add a sheet of 3/4" plywood over the bed mat to get enough clearance between the cab marker lights and the bottom of the cabin over bed on the Lance. It was close without the plywood and just the mat, doable but very close (within 1"). The extra 3/4" was enough added clearance to allow for any cab / bed articulation without hitting the lights against the camper.
My feeling is close is good from a turbulence standpoint (a wide gap invites wind resistance but you do need some clearance. How much will be determined by your personal comfort zone.
