My LEDs are mounted in my canopy are sitting about 5 inches above the water line. Other's mount theirs 10" - 12" above the water line. That gives you room to work in the tank if you don't have a way to raise/lower your lights.
My rock is on the glass. Been there for 9 years now. Some people make pvc
platforms, some put egg-crate under them. Egg crate is probably a bad idea cause it could cause dead spots with no water movement under the rock.
Have the fun of aquascaping is placing the rock so that it just rests on the rock(s) below and makes a solid wall with caves. There are so many areas in my rockwork that I would not see my mandarin for months at times. Found out when I was working from home one day that he did come around the front of the tank in the afternoon for a bit. But at lights out or company over, half the fish hide in the rockwork ad wont come out till the room settles down. On the other hand, when I get home from work they follow me begging for food... LOL.
Now get that rock in and placed and start filling that tank. Oh, do you want me to send you the rubber duckie I used while waiting for my uncured LR
to cycle my tank?