The set up is simple. Photon 48 (version 1, 90 degree optics) sits about 13" over the water surface on top of my canopy. The distance to the rocks, I am unsure about, but its well over 24". It's probably close to 40" to the bottom of the tank.
It might help to know that 90º lenses create a spread that's equal to their height.
So 13" off the water equals 13" of spread at the water surface. If it's 40", then there's a 40" spread. (Consider the tank walls as mirrors to get an idea of what happens to the "extra" width.
For an example, here's a diagram I did of Kessils where you can see the effects from the reflection (only roughly accurate):