Start with a lux meter so you can tell how bright the lights are - whatever you end up with.
With it, you'll be able to determine how high/low to mount your lights or where to set the dimmers instead of guessing.
Start with a free lux meter app for your smart phone, but a $15 handheld is a great investment. Useful for every lighting setup or adjustment, as well as around the house for other purposes! :)
You want about 40,000 lux at the water surface.
Anything from 20,000 to 80,000 is probably fine.
For what it's worth:
I have a tank that's probably closer to 10,000 lux than it is 20,000 and it's still fine. I'm guessing it's growing slower than the other (brighter) tank on the system, but I couldn't swear to it.