@Sexytang two answers for you. First, I’m pretty sure there is a BRS video on “optimal” mounting height for your light. It focuses on getting the light intensity as even as possible throughout the tank without a ton of spillage outside the tank. Second, there’s the practical answer of whatever works in your space. In either case, I’d use a PAR meter to setup and map out your light levels.
I run mine, 7 of them 8" off the water on both systems.
A par meter is a good idea.
Your settings can be adjusted up if needed.
I run all mine at.
1st 4 @ 100%
Remainder at 50%
Those setting will grow anything you want, imo.
This will give a very halide look.
This is a 30g running 2 16hds.
Tank is 24x20x18.
Quite a different look as compaired to the radions.