well, if it was a reef tank, you'd want your rocks to be lower in the tank to allow room for growth, otherwise you'd be growing out of the tank sooner or later.
since it's FOWLR though, what you have, is how it will remain, so it'd be a good idea to build it up higher, and have more volume.
there's some cool ideas and tips
here.
here's a pic of mine. they're not connected, though I SHOULD have connected them. you can stack the rocks pretty high as long as you build a decent foundation at the bottom and taper as you work on up.