How much bed clearance is enough?

[H. John Kohl]

I have the snap down tonneau cover so my bed rail is raised about 3/4 of an inch. The end by the tailgate gouged the vinyl under the bedroom area on a corner and inclined hill. So today, I just dropped or extended the hitch on the trailer to the lower holes and added about 2 inches. I would say a minimum of 6" bed rail to bottom of bedroom over hang. I have already raised the Reese hitch and do not want to take it higher because it will be above the bed rail and not allow the tonneau to cover it.