Here's my two cents on the idea of raising your rock: The idea originally (I believe) came from the Jaubert "Moncaco" style of reefkeeping where you would have a deep sand bed with a plenum but leaving most of the sandbed exposed. The idea behind the system was to have organisms in the sandbed "pump" nutrients down into the deep sandbed where there was little oxygen and anaerobic bacteria would break down these wastes (NO3) into Nitrogen gas. So it was necessary to have a large part of the sandbed exposed in order for it to work properly.
If the reason you're wanting to lift up your rock is to do this, then an inch or two would be fine. If you're doing this for extra circulation around your rock then I recommend you just be creative about stacking your rock. I have the exact same tank and what I did was to create lots of caves and space between the rocks on the lower levels. Obviously you want it to be stable so big rocks go on the bottom. It's actually recommended to place your rockwork first then pour your sand in around the rock because this would stabilize the rockwork and prevent shifting after it's full of water.
The point here is that you actually WANT less circulation in some areas. You want there to be a gradient of oxygen levels because it's in these areas where de
nitrification takes place. It's a delicate balance for sure. Besides, who wants to look at egg crate and tubing?
lol. Keep good circulation throughout the tank and behind the rock and you'll be fine.
Hope this helps.
