nruller said:
I would like to keep enough clearance so I can carry something on the lid if I wish. I figure that if I go with 8 - 10", I can get various things under for storage if necessary...
I'd suggest you limit the weight of any items you want to carry under the roof rack. With a RTT you already have 110 -125 lbs of weight plus what ever you can stuff underneath.
We had a customer who ordered extra high cross bars and a slide for under a roof rack. He managed to cram all sorts of things in there, tables, chairs, awnings, fishing rods, fir wood etc. I think we are all guilty of overloading trailers, stuffing things into every conceivable space.
In the end he put so much weight up there it racked the lid even after extra gussets were welded in. It also raised his center of mass considerably.