How much bed clearance is enough?

[HughM]

Just got our 2007.5 Chevrolet 2500 crew cab short bed duramax. Yesterday had CW install a Reese 16K slider. Today hooked up Montana (didn't pull it yet) and had 6 inches of clearance from top of bed rails on side of bed to the bottom of Montana.
Remember the Chev. bed is deeper this year.
The top of the Reese is 3 1/2" below the bed rail.
Reese hitch can be moved up 2 bolt holes and would give me about 2 more inches for a total of 8" trailer clearance and 1 1/2" below rail.
As a last resort I would drop the extended hitch on the trailer.
Question...it just doesn't look like its enough room in case I'm going up or down a hill with trailer on an angle.
I want to make sure I don't have bed rail and Montana meeting with a disaster in the making.
How many inches do you have between your Montana and your bed rails?